Each faction has a towed anti-tank gun available from its Gun Yard. These vary from faction to faction in range, penetration capabilities, strength and accuracy (as well as cost).
US M5 3-Inch Anti-Tank Gun
The M5 anti-tank gun was the United States' response to the increasing size of German tanks, and the first real anti-tank gun available to American forces. However the 3-Inch was regarded by many to still be insufficient, as it lacked the power and range to effectively deal with heavy German tanks like the Tiger and Tiger II, really only capable of doing serious damage to these targets with close-range flanking shots. Besides against the heaviest combat vehicles, the 3-Inch is still an adequate anti-tank gun and can be used to great effect against anything up to a medium tank.
German 75mm Pak 40 Anti-Tank Gun
The Pak 40 is essentially the same weapon featured on a variety of German vehicles such as the Marder, StuG III and Panzer IV. While Germany had more powerful anti-tank guns -- notably the 88mm -- the Pak 40 was still more than capable of dealing with most tanks. With similar capabilities to the US 3-Inch it is easily able to deal with medium tanks but will have trouble stopping heavy armour like the Soviet IS-2 or ISU-152.
British 17-Pounder Anti-Tank Gun
The 17-Pounder is the most powerful of the towed anti-tank guns. It combined long range with excellent armour penetration, able to effectively deal with all but the heaviest tanks at its longest ranges. It is also the biggest of the anti-tank guns, and as a result has a harder time tracking fast targets.
Soviet ZiS-2 Anti-Tank Gun
At 57mm, the ZiS-2 is the smallest of the anti-tank guns. However it is an incredibly efficient tapered-bore design, which results in an extremely high projectile velocity which mostly closes the performance gap with other anti-tank guns. It is able to adequately deal with medium tanks but, like the American and German guns, will have a hard time dealing damage to heavy armour.
While considering their individual differences, use of towed anti-tank guns is essentially identical for all factions. Anti-tank guns, when requisitioned (built), come in towed form. In this form the gun is highly vulnerable to even small-arms so must be protected, and generally only deployed in secure locations.
When deploying anti-tank guns it is important to take into account several factors. First is its firing arc or cone; when the Deploy button is clicked on a selected anti-tank gun a circle will appear, denoting the weapon's range (when deployed), as well as a directional indicator (orange lines) which show the arc the weapon will have -- moving the cursor around will change the direction the gun is deployed in. Because this arc is relatively thin, it is best to deploy anti-tank guns in groups of two or more to cover a wide overall angle. Also be sure to include a source of logistics nearby -- towed guns can carry only a couple of shells and must be constantly resupplied.
When deployed, anti-tank guns will provide a relatively well-protected defense against enemy vehicle and tank incursions, but will be unable to fire at enemy infantry as they lack high-explosive ammunition (and trying to directly hit a moving infantryman with a manually-aimed gun weighing upwards of 1000 lbs can be quite difficult). Because of their stationary nature, anti-tank guns can often have a relatively "short" life-time -- once the enemy spots where they are he is sure to try and take them out with long-range cannon and artillery fire, aircraft or infantry assaults.