What exactly is a single "atom" in a standard stream in linux?

Conceptually, a stream is a sequence of "characters" or "atoms", i.e. a binary stream is a sequence of 0s and 1s. But in Linux standard streams, if I write a bash script that asks "read", then I think it treats a single line (ending with "ENTER") as a "character", but I'm not sure. This suggests to me that a single "atom" is a string, and that atoms are delimited by ENTER. Also, I assume that for other programs, they don't take strings as input, but other data-types. Am I on the right track? what are the atoms/characters in a standard stream and how does the program know how to carve up a file into atoms?