Both Johnson and Clinton were impeached (the House does the impeaching), neither was convicted.
The difference that many people cite is that Clinton is the first and only ELECTED president to have been impeached. Johnson suceeded Lincoln, and the articles of impeachment against Johnson were more politically based than the articles against Clinton. Clinton was impeached because he broke laws invovling fundamental principles of civil law (lie to a judge), Johnson was impeached over political activities.
Yup .... another tid-bit is that BOTH impeached presidents were Democrats! ;-)