Scout
When the flag has to be there on time
Advantages
Speed, speed and more speed.
Disadvantages
Armour (minimal) Firepower (minimal)
Health
90
Armour
50 (spawns with 25)
Typical role
Flag carrier
Notable Weapons
Crowbar
This can be deadly in the hands of a scout, since most players' defence against the crowbar is simply to run away - but it's difficult to outrun a scout. The shock of being attacked by a crowbar-weilding scout can also surprise some players enough that the scout can get two or three blows in before any return fire.
Nailgun
Can be effective against Sentry Guns. If you can get line-of-sight without being smeared all over the wall, that is.
Caltrops
Anyone who treads on one will be slowed down to half speed, as well as taking a hit. Useful for hindering pursuers.
Concussion Grenade
As well as concussion jumping, these can be useful for removing enemies from your path - making them "depart by airmail".
Offensive Capacity
With his speed and agility, the scout is adept at making flag runs through a poorly-organised defence. He can regard a badly-positioned HWGuy as nothing more than a piece of the scenery, since he will be gone before the HWGuy can wind up his Assault cannon.
On territorial maps such as CanalZone, one or more scouts are an essential component of any team - they can get the flags to command points far more rapidly than any other class.
Defensive Capacity
Um. Not much, since attackers can kill a scout almost by sneezing at them. The most effective defensive use for a scout I've seen is to use the crowbar - get behind the enemy and hit them repeatedly.
How to fight them
Hit them with explosives in a confined space - their lack of health/armour means thay can't survive many hits.
The accuracy and firepower of Sentry guns make them very effective against scouts.
Block their way. A HWG in a strategic doorway can really mess them up. If a scout gets past you, there is no way you'll be able to catch him - your only hope is to hit them with accurate shooting before you lose sight of them completely.