The SU-76 is a light assault gun, produced by the Russian vehicle and self-propelled artillery yards.
The SU-76 grew out of the same program that spawned its big brothers such as the SU-122 assault gun. In November 1942, the Soviet Army decided to begin development of mobile assault guns to support infantry units. Previously, Soviet infantry had relied on field guns such as the light ZiS-3 and heavy M-30, which were towed by trucks, took time to set up in fixed positions, and were vulnerable to enemy fire. The SU-76 design took the chassis of the T-70 light tank, lengthened it by one road wheel, and replaced the turret with a casemate structure open at the top and rear housing a ZiS-3 76.2mm field gun. After some initial teething troubles with the engine layout in the first 320 units, the definitive SU-76M variant saw wide service in the Soviet army. Although crews disliked the open superstructure, leading to derogatory nicknames such as "Suchka" (Little B***h) and "Golozhopiy Ferdinand" (Bare-Arsed Ferdinand), the vehicle was light and maneuverable with a low ground pressure, allowing it to fit between urban structures and pass over ground too soft to support tanks. Infantrymen had a much higher regard for the vehicle than its crews; the 76.2mm gun was powerful and versatile enough to deal with anything from infantry and buildings on up to medium tanks and the open crew compartment allowed infantry to coordinate with SU-76 units without the need for radios. Some 14,000 were produced during WWII, and some examples were given to North Korea for use in the Korean War.
The SU-76 is about the most useful product of the Soviet vehicle yard. It grants medium tank-level firepower in a fast, cheap chassis relatively early in the game and can fire both HE and AP rounds. Most tanks are immune to the SU-76, just as the T-34-76 tank, but anything from a light tank and lower will likely fear the SU-76. Large numbers of SU-76s can hold the line against enemy infantry, vehicles and light armor until a Soviet player can produce T-34s, relegating the T-60 and T-70 to scouting missions. Later on, they can be paired directly with infantry for flanking attacks. However, the fixed gun, thin armor, and open top and rear are drawbacks, and stripped of infantry support a few grenades will messily end the vehicle.
In most situations you will favor the SU-76 over the tankette due to simplicity of use, but lack of accuracy and armor may call for a situation when fast, risky tactics will call for the relativly heavely armored T-60.