Male guinea pigs, also known as boars, have something females don't, a penis and testicles. That's right, if you think your safe from grossness, you are so wrong. Males need to have their penises checked and clean at least monthly. Anal sacks, which have glands for sent marking, can get badly mucked up from rubbing their bums on the ground to scent everything. Hay, hair and well anything you can think of can build up inside the anal sack and penis area.
When you tip you guinea pig up you will see the folds of the anus. If you gently pull the folds apart you should notice two parts:
The PERINEAL SAC which is actually 95% of the area and contains a sticky fluid produced from two small glands, this looks kinda white in my opinion.
The RECTUM which is a small opening where both the caecal pellets and normal fecal pellets are expelled from the digestive tract.
In older boars, (rarely younger and rarely in females because the sac is much smaller) the muscles of the anus stretch/weaken and the boar is no longer able to properly expel the fecal pellets from the anus. This can cause something called impactions, read more here: http://canadiancavy.webs.com/agingcavy.htm
A Regular Routine (no matter what age)
The perineal sac is a collecting place for all sorts of debris (hair, cage shavings, hay) and should be cleaned out on a regular basis. Mineral oil and Q-tips work well for this procedure. Your guinea pig won't like this procedure but it doesn't hurt as long as you are gentle, though the smell can sometimes be quite strong! It's possible, in some boars that haven't been cleaned regularly to develop an anal plug - basically a solid mass of debris and grease which is stuck to the anal wall. This can be worked out using liberal amounts of mineral oil to release the edges of the plug without tearing the skin - this takes time and a lot of patience.
After examining the anal sack, examine the penis and clean any debris you find. The whitish material is called smegma and may be odorous. Use a clean q-tip with mineral oil to clean this as well.
Here is a video I made to help you with anal sack and penis cleaning:
Why clean the penis? Smegma can build up on the penis resulting in the inability to retract the penis properly. This can result in the penis hanging out, dragging on the ground and ultimately getting open sores on the penis. Here is a photo of a 3 year 8 month old boar that was not routinely check and ended up with a minor mound on his penis. NOTE: After a good cleaning with mineral oil and daily application of polysporin his penis is now perfectly fine.