Direct Messages are messages that are sent to directly to one person. The conversation that you started remains between the two of you, and you can't add other people to the conversation later.
Spaces work well for a group of people working on a specific topic. This could be used as a collaboration messaging/meeting space for long or short time projects.
Teams are great for a consistent group of people who are going to be working on something for a long time. That something can involve many activities, or could be subprojects happening at the same time.