Scrum is one of the first-generation Agile systems designed for IT development projects with a single team of up to 10 team members. In one sense, it was more or less a lightweight management layer built to provide the necessary elements for the commonest Agile system of that time, XP (eXtreme Programming), which was focused on the technical aspects only. However, it gradually evolved from that initial state and became a standalone system.
Scrum provides a well-formed, simple system for some projects and provides them with what it considers to be enough project management. Its positive attributes, along with various environmental factors, have made it the most popular Agile system. Nowadays, most Agile projects either use Scrum or one of the second-generation systems that are highly inspired by Scrum.