At some point after reaching Level 20, you will receive a call from Merryweather, informing you that you can now call for a backup helicopter. This can be done from your in-game phone; open your Contacts, navigate down to Merryweather, and select Backup Helicopter. It will cost you $5000, so I would only suggest using it if you expressly have need for it.
A Buzzard will arrive and circle around you for a while. It will attack any players you also attack. It can also be used in selected Jobs. Many people find it useful while playing Capture. You can deploy it around your base, and it will hover around, picking off any enemy players who approach close to you. It can also provide support in Contact Missions.
The video below shows you how to call in the chopper when you have unlocked it, and how not to use it! It was unlocked on the Xbox 360 version, but the process is identical.