Early Mars probes were heavily sterilized. Further studies have shown that Mars's surface isn't habitable, at least not in most areas, and thus a low risk approach has been developed, where certain sensitive parts are carefully controlled (Usually robotic arms), and missions to certain interesting places (Such as locations where liquid water is suspected) are more carefully monitored, but overall, the restrictions are much looser.
There are 4 key bodies (Aside from Earth) that have interesting conditions that might harbor life in our solar system. These are Mars, Europa, Titan, and Enceladus. These are all known to have liquids at or near their surfaces, although how much is debatable. Mars has been studied well enough to know it doesn't possess any risk except underneath the surface, so we aren't as worried about it.
Specifically, there are 5 major categories of planetary protection objects of interest, where Category 1 has very low chance of life, and Category 5 is a sample return mission. The Wikipedia article linked provides further details.