The term "Percolator" or commonly "Perc" is used to describe a type of bong that contains extra elements to aid in the cooling of smoke. 

This cooling is achieved by increasing the amount of water the smoke comes in contact with when travelling through the bong. A percolator may be something like a showerhead that turns the smoke into very small bubbles which are more easily cooled as these bubbles pass through water. We at OzBongs call this "diffusion".

What types of percs are there?

Percs come in more forms and designs than I can attempt to describe. Anything that decreases the size of bubble or increases the cooling effect of the smoke can be termed a "perc". Common types of percs that you will see are things like tree percs, showerhead percs, inline percs, honeycomb percs and countless others. 

Some manufacturers will design their own percs and give them their own names and some manufacturers will use their own names for generic style percs. Generally if a manufacturer designs a very effective and popular style of perc, both the design and often the name will end up being used by other companies (Ethically or not).

What are the upsides and downsides of percs?

The upsides of a perc bong are the obvious cooler smoke which is preferred. Well designed percs can also create a very smooth, low resistance draw and high filtering of the smoke.

The downsides can be more complicated though.

To start with, percs are harder to clean, its harder to get into this small tight spots, bits of herb or particulate matter can get stuck in those spots and as I can attest personally from experience, an insect can crawl in there and get stuck becoming a nightmare to remove. So yes cleaning can be more difficult.

A perc can also be poorly designed. If you buy a high end perc bong, you will get a well designed perc that will function superbly. Sovereignty, Toro, Mobius etc put a lot of time and effort into creating highly functional percs that make smoking a pleasure. 

Percs on low end budget bongs can however be hit or miss. This is not an attempt to put you off buying a budget perc bong, but probably a suggestion to keep it to a design that generally reliable, ie: an inline perc, showerhead, UFO, tree perc etc.

A poorly designed perc can create unpleasantly high drag, splashing and poor usability.

Should you buy a perc bong?

It's not for me to say, it comes down to preference. I would however suggest that if you are intending to buy a perc bong, spend a little more time thinking about the purchase. Feel free to ask questions and do a bit of research if you like, especially if you are new to bongs or buying your first one.