What is the name of the Technic pieces that snap together to make a track?
There are three types of chain/track that are similar to what you describe, none of which are currently available through the official Pick A Brick, though all are fairly common on BrickLink.


The smallest is the Technic, Link Chain
The medium sized is the Technic, Link Tread
The largest is the Technic, Link Tread Wide with Two Pin Holes


There are also several belt styled tracks that might fit your description, but it sounded more like you were referring to the modular parts.