Trellis Coded Modulation (TCM) is the combination of convolutional (or trellis) coding and modulation into one step. A 1/2 convulation code takes one bit and codes it into two bits. Modulation is the process of converting a signal into analog form via a sinusoidal carrier. During TCM both of these processes occur simultaneously.
The two functions that occur during TCM are a Trellis code and a constellation mapper. The Trellis code provides a convolutional coder of rate R=k/k+1. The constellation mapper is a M-ary signal mapper that maps M=2^k input points into a constellation of M=2^(k+1) constellation points. These functions can be seen in the figure below.