More infantry

The SMG, or submachine gunner, is exactly like a Rifleman expect he has an SMG with 300 rounds and no ammo pack. The SMG is a really inaccurate weapon, best saved for close fights. If you're attacking a bunker or defending a cp (control point, ie, the flag building), SMGs are ideal. Oh, just for jokes, SMGs have pistols. Don't bother with them.

SMG

LMGs, or light machine gunners, are exactly that. They have a light machine gun with 300 rounds of ammunition and a knife - that's it. They chew through ammo like Bricktop's pigs go through human flesh. A machine gun needs to be deployed - this means pressing V to go prone, aiming yourself in the direction of your target, and pressing B to deploy. LMGs deploy only on relatively flat surfaces, and they have a limited traverse. So if your target moves too far, you'll have to re-deploy. LMGs are good when defending or camping enemy depots. Fire in bursts to conserve ammo and maintain accuracy.

Machine Gunner-LMG

Grenadiers are like riflemen who fire rifle grenades. They're limited to 60 rounds of rifle ammo, which is still more than you're ever likely to fire. French and German grenadiers have 10 high explosive (HE) rifle grenades each, while British grenadiers have 6 high explosive anti-tank (HEAT) grenades. For a rifle grenade to be used, press B to put on the attachment, then R to load a grenade. Once a grenade is loaded, it must be fired before you can go back to rifle mode again. British grenades are very poor against infantry but have the capability to kill tanks. It requires a direct, perpendicular hit against a flat surface. Go for the weak spots that sappers go for. Rifle grenades can be fired from any position, but from crouched position they're fired at a very high angle, like mortars.

Sapper-Grenadier

Sappers are the mainstay anti-tank infantry and also the units which bring down forward bases. A Sapper has only 30 rifle rounds but 4 HE and 4 smoke grenades. He also carries 4 5kg (11lb) charges of TNT. These can be placed on the weak spots of tanks to kill them. This site has weak spots for all tanks. Place your satchels where the little package icons are on the tanks.

Anti-tank rifles have been introduced very recently and the verdict is still out on them. They have a poor rate of fire and the knockback from the gun is significant, taking out a lot of ATP (stamina, the stuff you use for running), and the noise they make is very distinctive and loud. Infantry and tanks from all around are sure to hear it.

You should definitely check out Snoop's infantry training videos even if you never plan on playing infantry. They give you a good idea of infantry tactics as well as their strengths and weaknesses. This helps you defend yourself from them as well as work with them.