Okay, this will take a while, you may want to settle in. I'm going to cover what Judo is, where it comes from, why it is considered a "go to" art when people start asking about self defence, what a martial art is, and why thinking things like "but it doesn't have striking and kicking" really isn't a big deal when things are understood properly. Ready? Okay!
Judo is the creation of Kano Jigoro, developed in the late 1800's from Kano Sensei's study of a range of classical (Koryu) Jujutsu systems, most notably Tenshin Shinyo Ryu and Kito Ryu, with Fusen Ryu also being a big influence (honestly, I see a lot of Fusen in Judo's methods, with things like the Koshiki no Kata and Kime no Kata coming from Kito Ryu). Kano took his understanding of these traditional methods, developed ways of training them by paring back the range of methods to a categorisation of various groupings of techniques (individual throws, for instance divided into Te Waza [Hand Techniques], Ashi Waza [Leg Techniques] and Koshi Waza [Hip Techniques] etc), rather than the kata-driven method of the Koryu forms, where the basic mechanics would be learnt, then applied through a combative sequence, designed to instill strategies and tactical usage of the techniques, rather than just "how to do this throw". By focusing on the technical aspects of the throws instead of the overall strategies and tactics, there could be a greater focus on economical and efficient performance of the throws themselves, which would then be tested in a free-form training method, refered to as "Randori" (pretty literally: "Capture within Chaos"). This allows the student to apply their throws against a resisting opponent, and develop their own tactics to apply the throws by chaining them together, faking, and so on. It is said that a Judoka, with their tokui waza (favourite technique) will know every way of getting to that throw from any position, and from any angle, as well as knowing each and every defence against it.
Judo also gives us the modern belt ranking systems (Kyu/Dan system). Basically, Judo became very popular (the old story was that Judo, using slightly different rules to other systems, would consistently win inter-school competitions [an old holdover refered to as Taryu Jiai], basically by using rules that limited the other Ryu's methods. What is far more likely is that, due to it's limited repertoire, as opposed to a Koryu teaching and transmitting method, and it's emphasis on performance and competitive aspects, Judo saw itself being adopted in High Schools, as well as being popular around the nation). As Judo was so wide-spread, that as Kano travelled from town to town to teach, he wouldn't know the students (Koryu systems are typically small, and tightly knit, so outward displays of rank aren't necessary, everyone knows what level everyone else is, and the instructor certainly knows what level everyone else is as well), so he wouldn't know how experienced they were, or what they could take, in terms of techiniques. So he drew from the game of Go, which uses a Kyu/Dan ranking system, and developed the coloured belts so he could immediately see what amount of experience everyone had when he trained them.
In terms of techniques, it is primarily focused on throwing, with some ground work, limb controls, and chokes as well. The Goshinjutsu (self defence) portion of Judo does include some striking, the Kime no kata includes defence against long and short swords as well, so such things do exist there, they just aren't part of the competition side of things, so they can be glanced over in a number of cases.
Okay, now to why it's lauded as a good choice for self defence, that primarily comes down to it being a very solid training methodology, applied against resistance (in randori and competition). Add to that the relative simplicity of the technical curriculum means that the art can be applied very easily, without a lot of confusion (internally) getting in the way. This is important, and we'll come back to it. Finally, the basic rule that if you throw someone hard onto a hard surface (let's say concrete?), then they tend to not get back up again.
Now onto what a martial art really is, because this is important for the next part.
A martial art is a congruent philosophy expressed through combatively themed techniques and training methodologies. That philosophy could be physical, political, spiritual, cultural, or anything in between (remember, a philosophy is really just a collection of self-contained values and beliefs). And that philosophy, as I said, is expressed through training methods and techniques.
Now, if you come across a system that seems to only really focus on striking (boxing, muay thai, etc), or grappling (Judo, Aikido, BJJ etc), or even a particular weapon (Hyoho Niten Ichi Ryu Kenjutsu, Owari Kan Ryu Sojutsu, Ogasawara Ryu Kyudo etc), these are what are refered to as "specialist systems". In other words, they specialise in a single, or limited approach, chosing to become superior in that area rather than spread themselves across a more far-reaching scope. The other approach, of course, is to be a "generalist system", which does cover a wider variety of ranges (striking, grappling, ground, weapon defence and use, etc), giving a wider area that the practitioner is comfortable in, while never being as good in a single area as the specialist is.
These days, people seem to not understand what this philosophy actually means. Ideally, the philosophy should provide all the answers, whether it is a specialist system or a generalist system. And that seems to confuse people, especially where specialist systems are concerned. There's a big trend out there to want to cover all the bases, so people try to look for disparate specialist systems that are the "best" in each of their areas, even though that idea actually goes against the teachings of that type of system. Basically, a big part of training in a specialist system is that you are training to be able to apply the approach of that system in all circumstances, in other words, if it's a grappling/throwing system, then you should always be moving into a position where you can apply those grappling/throwing skills. Adding another specialist system is like running two computer programs at the same time, one saying you should open every window, the other saying you should close every window. Essentially, if you have a training methodology that says you should always grapple, and then you also train a specialist striking system that says you should always hit, when faced with a situation, which one do you follow? You can't always strike and always grapple, so one of them has to be ignored. And what that means is that the time spent training it is rather wasted, in all honesty.
Basically, what happens in a high-stress situation (such as an assault) is that your conscious mind basically shuts down, leaving your unconsious in charge (which is where all your survival skills and traits live). And the unconscious mind will always choose the best of any two options presented. Basically, it will look at what you unconsciously believe to be the most powerful, and go with that. Which means that none of the training of the system not classed as "more powerful" is basically ignored. And if the two conflicting systems are both seen as being equal (typically that the unconscious doesn't actually class either of them as sufficiently powerful), then you get an internal logjam (the confusion intimated earlier), resulting in nothing coming out. Really not a good situation.
For the record, a generalist system (such as the Ninjutsu organisations, certain modern and older Jujutsu or jujutsu-related systems, many Chinese systems, and more) most typically seeks to control things to move the opponent (say, a specialist) out of the area they want to be in, and that is how a generalist system works. It is not a collection of different specialist systems, as that is counter-productive to the way the specialist systems actually work. You simply cannot take two contrary systems and think they make a whole, as they both are already. Here's a food analogy: Creating a gourmet meal by mixing different high-end dishes in a single bowl doesn't make a great meal, it makes a rather bad experience. Stick with one dish in it's intended form, it tastes a lot better (a generalist system is more like a Yum Cha meal, or buffet, rather than a full collection of meals put together).
Hope that helped a bit.
