The Communists' Red Army (later the People's Liberation Army) had first been "formed" in 1927 when the Nationalist 27th Division mutinied. After ten years of fighting, though, in 1937, Japan invaded Manchuria. To counter this threat, the Communists and Nationalists formed the "United Front". Because of the lack of anything else, most Communist forces wore Nationalist uniforms.
Mainly, the two main Communist forces were the 4th Army and the Eighth Route Army, called such after their nominal designations in the Chinese national army. Consisting of three divisions (the 115th, which was commanded by Lin Biao, the 120th, and the 129th), the 8th Route Army's main advantage during World War II was its ability to blend in with Japanese forces when behind enemy lines. Also, the Communist forces could easily attack Japanese supply lines and small garrisons.
At the start of World War II, the 8th Route Army infiltrated itself behind Japanese lines in Northern China, where it established many bases of operations. It recruited Chinese semi-trained militia, and because of the long distance of the Communist bases from the large cities (which Japan focused its garrisons on), it was not economical for the Japanese to put much serious effort in attempting to suppress the Chinese forces. The lack of decent supply caused the Chinese to have to gain equipment from anything the enemy dropped. After World War II, the 8th Route Army had no serious combat. However, before and during the Korean War, the Korean People's Army (i.e. the Communist army of North Korea) was formed mainly from Korean veterans of the Soviet Red Army and the 8th Route Army.