Multi-band Compressors.

[KrazyKatz]

1 - Are they the same as a regular compressor except that they can delineate within set frequencies, or is there more to them?

2 - When would one generally use one & what tips could you provide?

3 - When would one definitely need to use one?

[Sam_Zen]

I didn't know they exist, but I guess it's an equalizer structure with frequency bands, where each band has its own compressor circuit.
Compression means loss of dynamics. Peaks are limited, soft sounds become louder.
Sometimes one has to use compression, with some microphones for example.
When to use it, depends on personal goal, I think. If you don't care about the dynamics, but just want to normalize a song as loud as possible, use compression. So I would say, use it carefully. Starting with a low percentage.
A multi-band would be useful to correct some resonation in a certain frequency range, leaving the other ranges unchanged.
A good compressor also has the opposite function : expansion.

[LPChip]

A dynamic compressor is not like an EQ.

It compresses a given frequency range different than another frequency range. It allows you to boost a certain frequency without it causing to clip. But in result, it can become less crisp (more muddy) so its often not wise to use this, unless you're sure its the best option to use.

[mrvegas]

A multi-band compressor\limiter is often used as the last effect in the chain in the master channel.  As you say, it permits you to compress different frequency bands different amounts.  In fact, the bass frequencies are often heavily compressed to prevent muddiness and keep the bass tight.  Then you can turn the output gain up on the bass and have a heavy bass without it sounding out of control.  In the commercial world, multi-band compression is pretty much always used.  The general wisdom is more compression on the very high and very low ends, and less in the middle.

Compression\limiting can be a complex subject and it is intertwined with EQ issues.  Do some searching on the forums at www.kvraudio.com and I'm sure you'll find plenty of information and links to free plug-ins.  I like Endorphin by Digitalfishphones, but there are many more out there.

Maybe I'll go on about this more another time, but I have to get back to work. Good Luck!