Afan Forest Park (aka Afan Argoed)

Easy. Get off the M4 on junction 40 and follow the A4107 for about 6 miles. Keep an eye out for the Afan Forest Park sign.
Or use this map to help you.

Afan has some extremely changeable weather patterns. Check out the weather in Afan here before you go.

The wall, 23 km of fast singletrack that got it's name from a totally awesome final descent. Sorry about the lapse into California-speak but you need a gnarly vocabulary to describe the twisty, rooty and well, awesome end to the wall. Leading up to it you can expect to find stunning views (a lot of the ride is completely exposed) and some quality technical sections. Its probably got the hardest climb of the lot but its worth it. After the 2003 refit by track designer Russell Burton, The Wall now links directly to Penhydd, so the truly committed/fit/insane now have over 40km of uninterrupted trail.
Time: 1.5 hours to 3 hours to complete it.
Class: Technically difficult.

Skyline is both the newest of the group and the longest. In fact its the longest single serving all weather trail in the UK. The cicular route is 46 km long with a huge 6 km final descent called Jetlag.
Epic is a word you'll often hear when Skyline is being described, due obviously in part to its length, but also because it has incredible views of the Beacons, the Black Mountains and the Preseli's.
This trail shouldn't be taken lightly, pack wisely (which your're doing anyway, right?) and take along enough fuel and sprares to get you to the end.
It goes without saying that you should be fit and have a bit of experience before attempting Skyline. If you are you'll be rewarded by a great ride.
If the weather isn't great, we are in a Welsh valley remember - they're green for a reason, then skyline is probably the most affected of all the trails in Afan due to its high level of exposure and length.
Time: 5 hours to 7 hours to complete it.
Class: Difficult

The oldest trail here Penhydd is also proably the only one to go for if you've not got much experience. Thats not to say that this 22 km trail is easy, or not worth the effort if your a grizzly veteran. Its got loads of fantastic tight singletrack descent, switchbacks and some dropoffs to test anyone. It was used in the 2001 singlespeed championships, so it's rated by those in the know as well.
You'll get a good mix of enclosed forest and the big views we associate with Afan on Penhydd.
Time: 1 hour to 3 hours to complete.
Class: Technically difficult

The majority of White's Level is technical singletrack, so with 15 km of the stuff it's suited for downhillers who know what they're about.
The 6km singletrack ascent (made more easier by the stunning views and clever weighted design) leads onto a trail that has got plenty of drop offs, berms and steep traverses.
Like the rest of Afan the trail changes a lot in the wet, but in the dry the surface is generally excellent.
RECENT UPDATE:
White's has been given a bit of a facelift, with the new 'Energy' section. Almost exclusively singletrack it pushes White's technical credentials. Berms, jump, timber boardwalks, bridge structures and a "general feeling of isolation." Cool.
Time: 2 hours to 3 hours.
Class: Difficult

This section is being written assuming you're a newcomer to DH and trail riding. We don't want any one coming back to us saying they got into problems because they forgot to eat - so we're telling you the lot...

1. A bike, down to preference. Hardtail or full suss. HT will get you up the hill quicker, but make it more difficult on the way down, both to handle and on the arse. FS will be easier on the way down and soggier on the way up. A full suspension with rear lock out is ideal, but take whatever you can.

2. A helmet. Take one.

3. Puncture repair/spare tubes/pump/tool kit/tyre levers. If you don't take them you know what'll happen.

4. Water/grub. Take as much water as you can, camel backs are very handy. eat like it's your last meal the day before (everyone always says pasta but just load up on any carbs - 2/3 carbs to 1/3 protein), and take some food with you. Sandwiches are fine, but cereal bars loaded with sugar are great. Buy them or even better make your own. Easy and full of energy.

5. Half decent clothing. The weather up there ain't exactly reliable. Some kind of weather proofed jacket - the more breathable the better (you'll get some sweating done), a base layer of some kind, again the higher wicking the better. Personally I'll usually just sweat through any old t-shirt, but my learned friends will usually take some proper logo packed trail shirt.
Most cycling trousers will wind you up on a trail, take some water resistant shorts.
Don't take anything you don't want to get muddy. Afan has been known to get a little grubby in the rain.
And gloves, you'll be in a world of pain the next day without them. Try and make them bike specific ones and not your 5.99 cotton jobbies.

6. Friends. Sounds daft but as well as being obviously more fun, you really don't want to get into trouble on one of the long trails on your lonesome.

7. Your mobile phone will only work occasionally and with a bad reception. Your decision whether or not to take it.

8. A camera. Take amazing photo's but risk breaking your lovely silly mega pixel digi-cam when you take a tumble. Again, your call.