Team Roles (UFO2000)
Assault is basically the guys who get up front to find the enemy units and kill him. This is the group that will move a lot around the battlefield, either chasing a retreating enemy, pressing an attack or defending a position. Ideally assault units should have a lot of TUs and a general purpose weapon, capable of auto fire and cheap (this last part for more strategic reasons: if the unit has a plasma rifle and it gets killed on the middle of the enemy....well you just gave the other player a plasma rifle). An Assault element can contain Scouts, Grenadiers, Rifleman and Assault units.
This element is responsible to provide either direct fire (against units spotted by the Assault/Security element) or indirect fire (against suspected enemy locations, to clear buildings, to respond to the enemy's fire). The main purpose of this element is to eliminate one or several enemy units on one turn. Therefore it should contain a mix of heavy (to clear enemy cover) and precise weapons (to take out enemy units). Correctly placing this element is essential to ensure its efficiency while supporting an attack. Usually the best positions are on open areas with no obstacles, which allow it to direct fire to all locations of the battlefield. However, those locations are also the ones easier to be fired upon. Support elements usually have Heavy Weapons and Rifleman units.
These units cover areas to prevent the enemy from moving there. The best example is when conducting an attack on one flank, where the security units will secure the other flank to prevent any counterattacks or to stall the enemy's movement. Effective security requires several factors: patience, to wait for the enemy to reach the position where it can be stopped (most of the times at close range); movement, to spot the enemy as it approaches and to fall back to a safe position; and firepower, to kill advancing enemy units quickly (and make the other guy slow his advance). Security units can be a very mixed bag: with the exception of Heavy Weapons, which might be too slow/inadequate to react to attacks, all other specialties can be used to this role. Sniper units can be very effective on this role but only relying on them can be VERY dangerous.
Most modern militaries use multipurpose fireteams, each one possessing a support weapon like a light machinegun and capable of assuming all of the roles listed above. This is the most flexible squad subdivision since it allows you to switch each team's role as necessary but it comes at the price of efficiency, especially if the teams can't properly support one another because of the terrain.