As basic infantry, riflemen often take the brunt of an assault. They are armed with assault rifles that just entered widespread use at the beginning of the war (AK-47 in the Soviet Union and the M16 among the Allies) and usually don't wear much protection - usually only fatigues and steel helmets.
As the weakest, cheapest unit, they are useful as scouts (due to their good speed), distractions, and as early warning sentries (since they can be posted in remote locations). They are susceptible to area of effect attacks when bunched up, and are weak to anti-infantry attacks in general. The damage they deal is so low that most vehicles can easily run them over should they hold their ground. Their weapons are quite sufficient at killing infantry, however.
When buildings are sold or destroyed, they often spawn a few rifle infantry. These infantry function identically to those trained normally, though rifle infantry spawned from destroyed buildings are damaged by default.
Red Alert Universe
Although the game manual says the Allied soldiers are armed with M16s and Soviet soldiers with AK47s, the game code identifies both weapons as M1 carbine, which would also be more realistic for the time period. Allied riflemen in cutscenes, however, use the M16.