Each and every talam has a structure, that is governed by the rules pertaining to it. For example, if we take the most common tala - Chatusra Jaathi Triputa Talam (Adi Talam), we can describe the process of the tala thus :
1 beat of the palm of the hand on the thigh,
followed by counting three fingers, then beating the palm and turning it over, then beating the palm and turning it over
If we count a number each for every beat, fingercount or turn of the palm - the number comes to eight. So the tala has eight units. The units are called Aksharams and the Adi talam has 8 aksharams.

The first part of the tala which consists of the beating of the palm & counting is called Laghu. Here the number of units is 4 (Chatusram) and the laghu is Chatusra laghu. The Jaathi of the laghu determines the jaathi of the tala so the tala is Chatusrajaathi Adi Tala.

The next process of beat and turning the palm is called Drtham. It is done twice, so the tala has 2 drthams. Thus Chatusra Jaathi Triputa Talam(Adi) has one Chatusra Laghu and two drthams. The symbol for laghu is 1 and the number written beneath it represents the jaathi and the symbol for drtham is 0 and hence, this thalam will be represented thus

Thus, from the above, we understand that a tala has laghu, drtham and these are called Angams (parts) of the tala. In some cases, there may be half of drtham, i.e., just the beat of the palm without turning it and is called Anudrtham.