Tumblr is a platform for young people to express themselves, discover themselves, and connect with people who share their interests. They generally use it to share videos, photos, links, music, etc. It is largely dominated by visual content and encourages the sharing of GIFs.
Think of it as a photo/audio/video clips streaming-album. Once they create an account, users can keep their content private, or share them with a larger audience. However, when it is created, the account is opened to the public. Users can subsequently edit privacy settings after the account is valid and created.
While Tumblr encourages creativity, it may sometimes contain pornographic or violent content.