There are 52 Guard Posts along the roads of Far Cry 2. Guard posts are like hardened checkpoints with turret guns. These checkpoints are guarded by a handful of enemies, often 3-5. They will sometimes have lookout towers from which a guard may shoot even rockets and usually have at least 1, if not 2 or 3 vehicles parked there.
Guard Posts will show up on your map as locked until you "explore" them. Exploring a guard post is nothing more than finding the weapons cache, explosives cache, or first aid kit in the guard post. Unfortunately, some of the guard posts have the "cache" in less than obvious locations which may make it a frustration to try to find them. The premise behind these guard posts is that you clear them and then use them as places to reload on supplies, however, an unlocked guard post will respawn its enemies at a frustratingly frequent rate (perhaps no more than minutes after being cleared, they will be fully respawned). This also adds an additional layer of frustration as you may find yourself clearing the same guard posts over and over and over again.
One way to avoid clearing guard posts is to try and drive through them quickly. The guards will hop into their vehicles and pursue you. Drive a small distance from the post and then switch to the mounted gun on your vehicle and finish off your pursuers.