A male cow is called a steer if it has been castrated and is called a bull if it is still able to reproduce. Due to the loss of their testicles, steers often exhibit physical differences from bulls, including less muscle around the neck.
When the testicles are removed, usually in a young male calf, it causes a reduction in the production of the testosterone hormone. In general, this makes steers far less aggressive than their bull counterparts. Steers can typically be kept in herds, while bulls should be kept alone, or they will fight for dominance. Both bulls and steers are killed for food, but the larger bulls yield more meat.
"Bullock" is a term used for a specific sect of male cows, but its exact meaning can vary amongst different types of English. In British English, "bullock" refers to a bull that has been castrated and is no longer able to reproduce. In North American English, "bullock" refers to a young bull (generally under the age of two) that has not been castrated and can still produce offspring.
Technically, the term "cow" refers only to an adult female cattle. "Calf" is the correct term for baby cattle, regardless of gender.