Fire Rate

Rate of Fire (or Fire Rate) is a description of the ability of a weapon to "shoot a lot of bullets in very little time".

Numeric Rate of Fire
Every weapon has a numeric Rate of Fire. This number more or less corresponds to the amount of bullets that can be fired in 1 second. A Sniper Rifle with a 0.4 Fire Rate will fire 1 bullet every 2.5 seconds. A SMG with a Fire Rate of 4 will fire 4 bullets per second. This rate does not take into account weapons with several bullets per shot (shotguns, for example)

Semi Automatic vs Full Automatic
A Full Automatic weapon will re-fire as soon as possible, so long as you keep your finger on your trigger. In this sense, it will make full use of it's rate of fire.

A Semi-Automatic weapon will only fire a shot when you pull the trigger. This will give it an overall slightly slower Rate of Fire then it's numeric capacity, as you will always fire a split second after the weapon has fired.

Burst Fire
Weapons with burst fire have a very miss-leading Fire Rate. These weapons will fire several bullets in quick succession every pull of the trigger, and then stop. The Fire Rate will give you no indication of how long this stop will last.

A typical example of this are Assault Riffles. They will fire in short bursts of (usually) 3 bullets, and then stop. The Fire Rate on these weapons describes the speed of individual bullets during a single burst, but not how far apart these bursts are. So even though an Assault Riffle may have a Fire Rate of 10, if it can only fire a burst every 1 second, it's true Fire Rate will actually be 3.3. Unfortunately, there is no way to know this Burst Fire Rate, but to equip the weapon and try it.

This is true of any weapon with "increase burst fire count". The rate of fire will only tell you the rate during bursts, and not the actual rate between two bursts.