How are parentheses interpreted at the command line?

Parentheses denote a subshell in bash. To quote the man bash page:

(list)    list  is  executed  in  a  subshell  environment (see COMMAND
          EXECUTION ENVIRONMENT below).  Variable assignments and builtin 
          commands that affect the shell's environment do not remain in 
          effect after the command completes.  The return status is the
          exit status of list.

where a list is just a normal sequence of commands.

This is actually quite portable and not specific to just bash though. The POSIX Shell Command Language spec has the following description for the (compound-list) syntax:

Execute compound-list in a subshell environment; see Shell Execution Environment. Variable assignments and built-in commands that affect the environment shall not remain in effect after the list finishes.

A command list embedded between parentheses runs as a subshell.

Variables in a subshell are not visible outside the block of code in the subshell. They are not accessible to the parent process, to the shell that launched the subshell. These are, in effect, local variables.

See Linuxtopia - Chapter 20. Subshells