Short for "cardiovascular exercise," cardio refers to any exercise that increases the heart and breathing rate.
Typical examples include running, walking, cycling, and rowing. Cardio is usually contrasted with strength training exercises like push-ups, squats, and pull-ups. However, strength training and cardio need not be mutually exclusive — stringing together a series of resistance exercises with minimal rest can certainly meet the requirements of a cardiovascular workoutMission essential fitness: comparison of functional circuit training to traditional Army physical training for active duty military. Heinrich KM, Spencer V, et al. Military Medicine, 2012 Oct;177(10):1125-30.Effect of concurrent endurance and circuit resistance training sequence on muscular strength and power development. Chtara M, Chaouachi A, et al. Journal of Strength and Conditioning Research, 2008 Jul;22(4):1037-45..