The word tala means rhythm, and rhythm is a very important thing in Hindustani music. The word tala is also used to refer to a rhythm cycle. A tala consists of a fixed number of beats, divided into two or more sections to make a pattern. Some common talas are:

dadra - 6 beats, 3+3
rupak - 7 beats, 3+2+2
kaharva - 8 beats, 4+4
jhaptal - 10 beats, 2+3+2+3
ektal/chautal - 12 beats, 2+2+2+2+2+2
dhamar - 14 beats, 5+2+3+4
dipchandi - 14 beats, 3+4+3+4
tintal - 16 beats, 4+4+4+4

The most common tala in use today is tintal, and the second most common is ektal, but the rest of the talas are also used fairly often.

The first beat of a cycle is known as the sam, and the counterbalance beat, which usually falls in the middle of the tala, is known as khali (the khali for rupak actually falls on the sam). Songs usually begin on the khali, and the tabla starts playing on the sam.

Unlike Carnatic musicians, Hindustani musicians don't need to keep tala themselves; the tabla plays a recognizable pattern so that by simply listening to the tabla the singer knows where he is in the rhythm cycle. This becomes very important as during the vilambit part of a khyal, these cycles are stretched to many times their length (so vilambit ektal is 48 beats); it would be near-impossible to keep count of 48 beats and sing at the same time.

The sounds of the tabla are represented by syllables such as "dhin" and "tha" which are surprisingly close to the actual sound made; the pattern that the tabla would play for any given tala are memorized by the musician. For example, the pattern for medium-fast tintal is (a syllable per beat):

sam               |                   | khali          |
dha dhin dhin dha | dha dhin dhin dha | dha tin tin ta | ta dhin dhin dha

This type of basic bol (syllable) pattern is known as the theka. Tempo is specified in relative terms: vilambit is slow, madhya is medium, and drut is fast.

Sources: The Raga Guide, ed. Joep Bor. Published by Nimbus Records with the Rotterdam Conservatory of Music.