Once you have a region and the mark is active, here are some of the ways you can operate on the region:
Kill it with C-w (see Killing).
Save it in a register with C-x r s (see Registers).
Save it in a buffer or a file (see Accumulating Text).
Convert case with C-x C-l or C-x C-u (see Case).
Indent it with C-x TAB or C-M-\ (see Indentation).
Fill it as text with M-x fill-region (see Filling).
Print hardcopy with M-x print-region (see Hardcopy).
Evaluate it as Lisp code with M-x eval-region (see Lisp Eval).
Most commands that operate on the text in the region have the word region in their names.