Can someone explain to me in plain English what a Sprint is, and what the alternative or opposite is?

I swear that everything I search and look up is vague as fuck.

I worked in "Sprints" at an agency I guess? I mean, we planned out the whole project, then broke it down into smaller projects. the created tasks within those projects that had a time frame in which to be completed before going on to the next task. Was I sprinting?

Are Agile and Sprint just fancy as terms for the only way you should be getting shit done? Seriously.

What is the opposite of a sprint?