One big adreneline rush is one way to describe it.
I'll try to give you an overview.
There is one flag and two teams.
Each team has a goal to which they have to carry the flag to. This goal is usually located at the opposite side of the map from where you spawn.
The flag starts at and reappears after a goal at what is called McHome. McHome supposed to be located equidistant from each teams spawn point.
McHomies (McHome defenders) usually consist of Hauptmans, Daishi, Warthog,.....you get the picture.
Runners usually consist of Dashers, Fleas, Ullers and BlackHawks.
Chasers usually consist of Gargoyles, Lokis, and sometimes Dragons.
The object of the game type is to gain control of the flag and run it to your goal. Easier said than done.
McHome is usually being fought for by big ugly assault type mechs that just love to see light mechs run in front of them. It's not unusual to get 30 kills as a McHome defender. You can also get killed 30 times as a McHome defender. As a McHome defender you want to try to time your push for McHome so that when the flag is capped at a goal you have control of the area around McHome and can pick it up when it reappears.
As a runner your only mission is to take the flag to the goal. You don't have much in the way of weapons because your mech should be at max speed. Speed is life for the runner. You stay away from the bad guys unless you think there is a chance you might be able to pick the flag. As a runner you also have to be able to time your rush to McHome to pick the flag when it reappears.
The chaser can have two missions. Chase the opposing runner down and take the flag away from them and help out at McHome when needed. This is where the high speed Gargoyle is at it's best

The Gargoyle can almost run down a zigzaging Flea and still have enough firepower to knock it down.
There are alot of tactics involved with this game type. If you want to learn more read this
McRugby over at MWL.