If you're interested in finding more information than purely the location of a program, you might try the whereis program:

$ whereis ls
ls: /bin/ls /usr/bin/ls /usr/share/man/man1/ls.1.gz

Here we see that ls occurs in two common binary locations, /bin and /usr/bin. Additionally, we are informed that there is a manual page located in /usr/share/man. This is the man page you would see if you were to type man ls.

The whereis program also has the ability to search for sources, to specify alternate search paths, and to search for unusual entries. Refer to the whereis man page for further information.