Arathi Basin (AB) is a 15v15 node-based battleground. You can join as a group. If you are outnumbered, it does not matter, as the game will not attempt to balance numbers by preventing the opposition faction from joining. A minimum of 8 players is required for a battleground to continue, otherwise it will automatically end. However, because of the rate of points gain for 5 capping, you should be able to finish before your time expires.
Aim
The aim of AB is to reach 2000 resources before the opposition. You achieve this by capturing nodes, which look like a flag. The more nodes your team controls, the faster your resources will increase. The table below represents this.

Honour bonuses are also given periodically in AB. Each multiple of 330 resources rewards 198 bonus honour. Additionally, winning the match also rewards 198 honour. On AB weekend, the bonus honour is accrued every 200 resources.
You also gain reputation with the Arathi Basin faction for your side (The Defilers for Horde or The League of Arathor for Alliance players). Increasing your faction with these groups unlocks access to some excellent items. Each 200 resources gives you 10 reputation with your respective faction, up to a total of 100 for a victory. On AB weekends, you get 10 reputation per 150 resources, which is 130 per win. You can also get 800 rep from the quest The Battle for Arathi Basin!, which requires you to assault 4 bases. You do not have to click the flag yourself, being in the raid and near the flag when it is capture will result in successful completion of the quest.

Strategies
There are a few various strategies for success in AB. To win, as stated, your team needs to be the first to accumulate 2000 resources. To do this, you don’t need to have all 5 nodes – a common mistake made by pub teams. You simply need to hold a majority of the nodes for more time than your opponents. Simple.
The Blacksmith is a vital node in AB. Make sure your team-mates realise this. You can quickly jump from the Blacksmith to any of the other 4 nodes, because it is centralised. If you control the Blacksmith, you essentially control the flow of the game, as you can deploy troops to the other nodes extremely quickly.
To counter the power of the Blacksmith, you can “trap” your opponents in there, though it can be hard to do. If you allowed them to capture it in the initial rush, while your team grabs the Lumber Mill and the Mine, then you can push from both sides, and if successful, effectively keep your opponent trapped in the middle while you hold at least 3 nodes, possibly 4. This can keep them occupied for at least a small time, while you accrue a small but effective lead.
Also important strategically is the Lumber Mill, as it provides an unparalleled view of the battleground. You can easily see where the other team are headed from the cliffs of the mill, which makes it very easy to counter offensives. The best combination is blacksmith, mill, and stables/farm. You can see all three from mill, dispatch quickly from blacksmith, and defend your third node easily.
A simple strategy that is almost always overlooked is to STAY WITH YOUR NODE! Most people will rush to a node to capture or defend it, and once the battle is over, will ride or run off to the next point. This is an easy way to give back the node to the opposition. Even two players left at a node can effectively hold off an attack until backup can arrive. If you leave it empty, a single player (which may even be a stealthed rogue who didn’t join a losing battle) can quickly undo all your hard work.

The most common tactic (and the least effective) at least in pick up AB groups is the zerg. Players will attack together with as many as possible, often leaving nodes undefended while they attempt to take as many as possible. This is generally a bad tactic, as it is easily counterable, but it can work. If you are a co-ordinate group with some healers and some well-geared damage classes, you can use this tactic to pin down your opponents. If you can keep pushing them back to their own base, you can 5 cap and cue up a quick win as well as some easy HKs as you keep them pinned in their spawn point.
Rewards
AB rewards are more suited to casters. The Ironbark Staff and Mindfang/Sageclaw both have +damage/heal and critical strike on them. There are also excellent cloth shoulders for caster classes.
For melee classes, there is an excellent cloak with agility and attack power bonuses. There is also a 3-piece set for each classes, with two blue and one epic piece. Only the shoulder pieces are really worth getting; the general honour sets have better stats. However, the blue pieces have lower level counterparts for each piece, so they are available from level 28, which makes them more attractive for the levelling PvPer.