whereis command

search in several location /usr/bin... and returns exec file and related resources such as man, config.

whereis sl
sl: /usr/games/sl /usr/share/man/man6/sl.6.gz

which command

only search in $PATH and only return path to exec:

which sl
/usr/games/sl

type command

return how a command is interpreted:

type -a echo
echo is a shell builtin
echo is /usr/bin/echo
echo is /bin/echo