Perk-a-Cola machines gives players special abilitys that are reffered to as "Perks". They can be very usefull in tricky situations where one would typically be overrun.
Juggernog - This gives the player extra health allowing them to take more hits from zombies and making it easier to escape from tight situation.
Speed Cola - This allows the player to reload at a faster rate, this is very useful in later rounds as speed becomes the main aim of survival
Double Tap - This allows the player to shoot any weapon at a much faster fire rate, same with Speed Cola, this is very useful in later rounds where speed becomes the main aim of survival
Quick Revive - In Solo, If the player is killed they will be given a second chance, however in multiplayer, This will allow the player to revive his team mates at a faster rate, this perk is normally over looked when your in a group of very good players
PHD Flopper - This gives you invulnerability to explosions. You also can't take fall damage with this perk, instead where you would normally take damage, you will cause an explosion that will hurt all zombies in range
Stamin Up - With this perk, The player no longer has to rest after sprinting and a much faster speed.