Rim locks are effectively any door lock which is fitted to the surface of the door rather than routed into the door itself. This includes your traditional rim nightlatches we're all familiar with (commonly known as "Yale Locks"), as well as surface mounted sashlocks and deadlocks still found on older properties with their original doors still in place.
When it comes to rim nightlatches, as times have changed, the complexity of security and diversity in functionality has somewhat increased, so where you used to just have your plain old rim lock with a snib to hold the latch back, you'll now notice a lot of deadlocking options are now available.
Deadlockable nightlatches in their basic principle, allow you to use your key when leaving the property to deadlock the latch, preventing jemmying/latch-slipping attacks which became a concern for these types of locks back in the day.
You then have auto-deadlocking nightlatches which host a separate deadlocking snib above or below the latch, which is compressed by the striker plate when the door closes, triggering the latch to deadlock in-place when the door has closed. This removes the need to use keys in order to deadlock the latch, as it's all done automatically as the door slams shut.
You'll also notice on some of these locks that there's a keyway on the inside handle now, not just on the outside - again, this is all about maximising security - so the old trick of breaking a pane of glass and unlocking the door by putting your hand through to the inside is now fiction with these particular locks, because you can double-deadlock the door, so the door is locked, the latch is locked, and the handle is also locked.