Colleagues: people you work with regularly, within your company. May be at different levels. Peers: people who are at a similar level of knowledge and experience as you, within or outside your company. Counterpart: a person who has the same job as you in another company or the person from another company whom you work with to get business done

Colleagues are the people you work with. Peers are people in the same group as you, who are at a similar level. For example, they could be the same age as you, do the same type of work as you, or have the same status as you. A counterpart is someone who holds the same position or performs the same duties as you at another location.

Very little. Colleagues are usually thought of teachers or educators in the same school. Peers are people of equal status or age. The word peers in modern internet slang has morphed into "Peeps". Counterparts is a bit different. I have to give an example. When I was in Vietnam I was a Signal Corps company commander. My counterpart was a Vietnamese Signal Corps company commander. We each commanded our respective commands, doing the same mission, and we were equal, but separate.