In the World of Warcraft, Druid, like Shaman and Priest is able to heal in PvP situation, but Druid is one of the most verstatile classes. This verstatilly does not serve a Druid as well inarenes or battleground, but it does make a few unique strategies available to the players of the class.
   The key to being effective as a Druid in PvP is using the forms effectively. The travel form, for example, makes a character immune to the Mage's polymorph. A Druid character that switches into the travel form in PvP before the mage casts polymorph gains control and can then use the cool down time to switch forms and deal a large amount of damage to the mage.
A Druid should keep the regeneration spells the class has access to up at all times in both PvP and PvE combat. In the battlegrounds and in arenas this gives him a little bit of extra time to get away. Root spells should also be kept on the hot key the player uses on other opponents. This keeps opposing players from getting at the Druid or in the case of feral characters keeps weaker opponents from running.
A Druid is often quoted as a jack of all trades, master of none, which is true. However, lately all three specs are making an appearance in arena. In general Restoration Druids are the highest rating Druids. Their heals are mostly instant cast dots – which means they can heal instantly and then no longer have to keep LOS for the healing to continue. This is a massive advantage, and indeed one of the advantages of all dots/hots. Second, travel form is the best anti-snare in the game, and all it takes is one lucky slip for a Druid to break distance, break LOS, and heal themselves back to full. Third, resto Druids have the greatest mana regeneration rate in the game. A resto Druid can keep the heals coming for far longer than an equally geared Paladin. Ferals and balance (moonkin) Druids also have places in arena, though their effectiveness is usually limited to the lower brackets.
Although Druids have limited crowd control in the form of Roots and Nature's Graps, they're awesome at countering enemy crowd controls. Take fighting a Mage as example: as soon as you see them casting polymorph (yeah, get a good add-on for the cast bar by the way) go into travel form (you could switch to any form, but travel is cheapest mana-wise) and you'll be immune.
Due to their high survivability and HP/mana cycle, PvP Druids are great at attrition fights. If you're feral, try to keep rejuvenation and regrowth on yourself whenever possible, then switch back to bear form and let your mana regen. True, it's not an endless cycle, because you'll eventually go OOM doing this, but it gives you enough time to chew on most classes.
One of the most fun facts I found while PvPing with my Druid was my ability to lure players into wasting their cooldowns and nukes. For example, most classes will save the nuke for when you're in human form, trying to heal yourself. If you go into human form earlier on (30% hp for example) you'll most likely lure them to try to finish you off with their nuke. Cast a fast heal and go to bear form immediately and start clawing at their face. It's funny to see them standing there, with their nuke wasted, while you're still with full mana and topping your HP bar as well.
One of the most interesting PvP battles I fought was against a fellow Druid. It took around 15 minutes and we both went on and off the edge, it was truly an epic PvP fight for me. He managed to score the final blow on me with as much as 300HP or something similar, but it was still an awesome experience.