Oh boy. You ready for this? Okay....
Jujutsu is a generic term first used to describe unarmed or lightly armed methods associated with the Japanese Samurai, although it was by no means an exclusive term. Other terms included wajutsu, yawara, yawaragei, te, karate, judo, taijutsu, hade, wa, goho, dakentaijutsu, katchu yawara, yoroi kumiuchi, kogusoku, koshi no mawari, and others. Add to this the fact that even if different systems use the same terminology, that doesn't mean the technical aspects are teh same, or even similar.
Being a Japanese system, there will be a prevalence towards grappling techniques (stand up, rather than ground work), although striking will also be taught, depending on the system itself. But again, this will depend mainly on the system itself.
Two main groupings to look at (in terms of the Japanese systems.... I'll deal with the other in a moment) are Koryu and Gendai systems. Koryu systems predate the Meiji Restoration (pre-1862), and Gendai systems are ones that came afterwards. Many Koryu systems may have a Jujutsu syllabus within their teachings (Katori Shinto Ryu teaches Yawaragei at a high level, Hyoho Niten Ichi Ryu has a Jujutsu syllabus at a high level, Yagyu Shingan Ryu has a large number of Jujutsu sections within it's teachings as well as it's weapon techniques) or are focused on Jujutsu as a primary teaching, often with weapons as well (such as Takenouchi Ryu, Kito Ryu, Tenshin Shinyo Ryu, and others).
The origins of these systems and their Jujutsu teachings often refer to application on a battlefield if you lose your weapons, although later systems (after the Tokugawa Shogunate took over around 1608), being removed from the use on a battlefield, focused more on the unarmed section itself. Some of these became "commoner's yawara, or jujutsu", being slightly simplified versions of the older samurai systems taught as a means of income to the samurai who no longer had wars to occupy them.
The Gendai systems often have their roots in Koryu (or Koryu-related) arts, such as Judo (from Kito Ryu and Tenshin Shinyo Ryu) and Aikido (from Daito Ryu Aikijutsu, with influence from Yagyu Shinkage Ryu Kenjutsu, Kukishin Ryu, and a few others) and so on. Another fairly commonly encountered modern system is Hakko Ryu, which has a few off-shoots as well.
A third variant on this idea is the Ninjutsu-realated systems, who teach a range of older jujutsu-style systems from a range of backgrounds, but in a Gendai integrated format (especially the Bujinkan).
Then you get your Western systems who use the name Jujutsu (or a variant of it). These include Danzan Ryu and Brazilian Jiujitsu (BJJ). BJJ is very well known these days, and comes directly from Kosen Judo (essentially, High School Judo) when Kano Sensei sent a number of representatives out in the world to spread his art, and one of the main proponents of Kosen Judo (Maeda) came to Brasil. He taught a family called the Machados, and their cousins the Gracies, the Kosen form of Judo, which emphasised ground work over throws and stand-up grappling. It evolved in Brasil, and became what is found now.
So, to sum up, Jujutsu refers to a Japanese or Japanese-influenced (or based) unarmed or lightly armed method of combat which is most likely grappling based, but may be more focused on striking, or at least integrate striking... then again it may have none at all! It really depends on the system itself. So, if you could provide that detail, we may be able to give you a more specific answer.
