Ninjutsu (忍術; Literally meaning "Ninja Techniques") vaguely refers to pretty much any technique that uses chakra and allows the user to do something that they otherwise would be incapable of doing. Unlike genjutsu, which makes the opponent see illusions, the effects of ninjutsu are quite real. Ninjutsu relies on chakra and, most of the time, hand seals (each hand seal showing a different animal in the chinese zodiac) to be effective. The hands are put in sequential positions that mold chakra in the necessary manner to perform a jutsu. This is not always consistent, however, and some of the simpler jutsu seem to become second nature to experienced ninja, after which they can perform them at will. Additionally, some ninjutsu can be classified as an elemental jutsu, in those cases where a specific element (wind, water, fire, earth, lightning) is used.
Genjutsu (幻術; Literally meaning "Illusory Techniques") are techniques that are employed in the same fashion as ninjutsu, requiring chakra and hand seals. However, the primary difference between the two is that the effect of genjutsu is illusory, creating chakra imbalances in the victim's brain, causing a disruption in their senses. This is often used to create false images or cause pain from trauma (because the body is led to believe it is in pain), however there are plenty of other uses depending on the situation. They are not used as often as ninjutsu because they don't physically harm the opponent, but several crafty characters have made themselves well known for their great execution of genjutsu. Genjutsu is created when a ninja extends their chakra flow through the cerebral nervous system of their opponent to control their mind's chakra, thereby affecting their five senses. Genjutsu is translated as "Illusionary Techniques"
To combat the effects of genjutsu, there are two options. The first option is for the ninja to stop the flow of chakra in their body, and then apply an even stronger power to disrupt the flow of the casters chakra. This can also be done by an unaffected ninja. The second option is for the affected ninja to hurt himself or herself, and then utilize the resulting pain to bring their senses back in order (of the two options, the latter seems more effective in most situations, or at least is the easiest). Genjutsu attacks usually will require a lot of skill, and if performed incorrectly will not work; even Jiraiya had trouble performing such techniques. These types of attacks use up a high amount of chakra control and need perfect hand seals. Because of this they're most often used by a fighter with very precise chakra control and very high intelligence
Taijutsu (体術; Literally meaning "Martial Arts") is a basic form of jutsu that generally speaking requires no chakra, but chakra may be used to enhance techniques. Taijutsu generally require no hand seals to perform and are much quicker to use than ninjutsu or genjutsu. Taijutsu is simply put, hand-to-hand combat or weaponry.
Although it is used by nearly all ninja, Rock Lee and his teacher Might Guy use taijutsu almost exclusively, and are experts on many taijutsu techniques. Example of Taijutsu techniques are Dynamic Entry and Leaf Whirlwind. The Hyuga Clan's Gentle Fist, however, makes extensive use of chakra and demands great skill in its control. The taijutsu techniques are the right choices for those that want to save their time and chakra, or for those who rely on their physical attributes.