Like other editors, Emacs has commands to search for occurrences of a string. Emacs also has commands to replace occurrences of a string with a different string. There are also commands that do the same thing, but search for patterns instead of fixed strings.
You can also search multiple files under the control of a tags table
(see Searching and Replacing with Tags Tables) or through the Dired A command
(see Operating on Files), or ask the grep program to do it
(see Searching with Grep under Emacs).