Volibear is a very unique champion in League of Legends because he can be build in a lot of different ways to augment your team and counteract your enemy's strengths.

Anti-Carry: If you see that the enemy team has a very strong AD Carry, you can build Voli to counteract their presence on the map. Especially when you're not the only tank. I prefer this build.

Mage Slayer: If you see that the enemy team has a very strong AP Caster, you can build Voli to counteract their presence on the map. Especially whey you ARE the only tank.

The idea is to be the catalyst in the team fight... Kill the AD Carry first and that is your job to focus them to the point of death. Volibear can be build to have a high damage output and can be very tanky. This build is especially useful when you have a 2nd tank on your team such as Nautilus, Shen, Taric, or Maokai etc. Soloing top is a great way to start off on this build... Use your turret to your advantage because the enemy team will get antzy and you can bait them into a "turret dive". My item build focuses giving me the most health to maximize the damage "Frenzy" (W) will do in a single use...

When facing strong AP Casters such as Malzahar, LeBlanc, or Xerath; you want to be able to not death from their abilities. With that said... purchasing magic resist and health regeneration makes you hard to kill and plus gives you team an advantage in teamfights.(To clarify: Start off with Cloth Armor and 5 health potions.) You won't be as "tanky" as my "Anti-Carry" build, but with the amount of MR and health Regen you have plus Voli's passive makes him hard to kill vs. AP Casters or any champion that uses ability power. This build focuses on using Voli as a sponge, soaking up all the magic damage so your team doesn't. Voli's Majestic Roar (E) and Lightning Claws (R) rank off of AP so Lich Bane, Rylai's Crystal Scepter, and the Abyssal Scepter help the damage output from them. IF a allied champion has an Abyssal Scepter then grab a Warmog's Armor.

Pros:
Volibear is hard to kill
He can surprise enemy champions by his closing speed (with his Rolling Thunder (Q)) and his damage output.
Using Rolling Thunder (Q) and Majestic Roar (E) in tandem is the perfect way to grab an enemy and pull him into your team.
Cons:
Volibear does NOT have a great escape planned... if you get away from a teamfight that got out of hand, well you're lucky.
Fear (crowed control) really hurts "Anti-Carry" Volibear because he it the initiator of the teamfights... getting feared pretty much means your dead...