Taijutsu is a martial art which may include strikes, kicks, joint locks, throws and many of the techniques found in the above mentioned (Aikido, Judo, Karate, etc), it depends on the particular ryuha. While most of its aspects appear external, the dedicated student will find many internal aspects as well.
Taijutsu is commonly associated with the schools that supposedly teach Ninjutsu, although there are many ryuha with no relation to Ninjutsu that have Taijutsu in their curriculum. Asayama Ichiden ryu is just one example, there are far too many to list them all. Taijutsu was used by Bushi/Samurai, some Ninja also used their own version of Taijutsu.
Currently, most Taijutsu schools operate under the X-kan dojo. There are, however, a few which teach lesser known stylings of Taijutsu.
A Taijutsu ryuha may contain anywhere between just one category to all categories of the list above and more.