Most kanji have at least two pronunciations, being kun'yomi and on'yomi. It comes from the fact that the Japanese written language is basically just copied from the Chinese written language, and applied over their own native words. As a result, the pronunciations are the way the Japanese pronounce the word the character refers to (kun'yomi, essentially "sound reading"), and the way the Japanese thought the Chinese pronounced the word (on'yomi, or "written reading"). To give you an idea, the character for "sword" is 刀, which is pronounced as "katana" in kun'yomi, or "to" in on'yomi. The Chinese pronunciation is "Dao". The other word for sword is 剣, which is pronounced as "tsurugi" in kun'yomi, or "ken" in on'yomi. The Chinese pronounce this character as "Jian". When you compare "To" to "Dao", or "Ken" to "Jian" you can see how that came about.
When it comes to 術"jutsu", though, there is only the one pronunciation given (the Chinese pronunciation is "shu", by the way). So it's always "jutsu", technically speaking. There are a range of words pronounced "jitsu", including 日, which means "sun", and is part of the name of Japan itself (Nippon, where the character has the pronunciation of "nichi"), but most commonly 実, which means "truth".
So, to sum up, 術 = jutsu (practical art, techniques, technical methods); 実 = jitsu (truth, real, actual).