Infantry includes all squads of foot troops, whether human or alien. A typical unit of troops is between five and ten warriors strong. Infantry can cross almost any terrain (given enough time) and make the best use of cover to avoid enemy fire. Most large bipedal creatures such as the Tyranid Carnifex are counted as infantry although they are usually individual models.
Infantry
Source: Core Rulebook 3E, p. 40
Unit type covering all squads of foot troops, human or alien; can cross almost any terrain and use cover well.