Well some of the enemy parties have units that you yourself can recruit and build up, so they're fairly obvious, but others are not. I'll write down a very rough guide, anyone else feel free to add to it.
1. River Pirates - Very weak enemies found near the river by Zendar. These units are never mounted, and carry very weak weapons (clubs and cooking knives quite often) and have very light armor. Perfect prey for the newbie adventurer.
2. Mountain/Forest Bandits - I haven't noticed any real difference between these two. They are typically found near the forests and stick to the middle of the map mainly. These are basically just higher level river pirates wielding slightly better weaponry, but still very weak armor. The most noticable advantage they have is that a few, about 20% of them, are mounted, but still not much of a threat.
3. Sea Raiders - These pirates from the sea generally roam near the oceans on both sides of the Zendar river. These are to be feared by early adventurers as while they aren't mounted, their infantry is quite devastating. They wear medium armor and typically wield powerful long reaching axes, both 1hand and 2hand.
4. Steppe Bandits - Generally found near the center of the map, these bandits tend to favor range over melee. They have a fair number of cavalry, about 40%, most of which are mounted archers. The rest of their infantry favor range, and will pepper you with arrows/javelins. Their armor is fairly weak, so they're easy to kill once you close the range.
5. Black Khergit Raiders - Generally found in the corner of the map near Tulga and Reyvadin, these units are all mounted. They favor a balance of horse archers and spear wielders with medium level armor for protection. As such these can be very formitable opponents for all but the best armies.
6. Swadian desserters - These units can be found near the center of the map, but do tend to travel a bit. The are comprised of nothing but crossbowmen. This makes for a fairly powerful force capable of tearing your army apart at long range. They are also fairly well armored and carry decent melee weapons, so be prepared.
7. Vaegir desserters - Found in the same locations as the swadian type, these tend to be more like slightly better mountain/forest bandits. These units are comprised entirely of vaegir veterans. About 20% are mounted, and they are slightly better armed and equipped than the bandits. Generally not as big a threat as the Swadian type.
8. Forager - Belonging to the Swadian and Vaegir nations, these can be found almost anywhere, but generally stick to the center parts of the map. They are mostly peasents escorted by militia and a few higher class archers. Overall they are pretty week and make good practice before fighting a real army.
9. Scouts/Patrols/War Parties - These units belong to the Swadian and Vaegir nations and as such can be found almost anywhere on the map. They have been listed progressively in terms of power and typically have a very large number of powerful mounted units supported by archers. Don't do battle with these until you're very sure of your combat abilities.
10. Dark Riders - The most powerful parties in the entire game. They can be found almost anywhere, but are pretty rare. They are comprised of dark knights and dark riders, all of which are mounted. Dark riders are pretty well equpped with decent weapons and armor, however dark knights are uber units of much death, doom, and destruction. They wear suits of black armor (best armor in the game), ride on powerful war horses or chargers, and have the best weaponry one could ask for, although they are strictly melee(no need for archery here). These are the ultimate challenge for any hero.
Also note that the foragers/scouts/patrols/war parties only become hostil once you ally with their enemy nation. So there's all the stuff i can come up with off the top of my head. Feel free to add/correct this list as you all see fit as i'm sure i've left stuff out.
