Google Drive

Google Drive is Google's cloud storage, integrated directly with Docs, Sheets and Slides so files created there live in the same place as uploads.

Its sharing model is the part worth understanding properly: link sharing, domain sharing and individual permissions behave differently, and the common accident is a file set to "anyone with the link" that then gets indexed or forwarded.

Storage is shared across Drive, Gmail and Photos on the same account, so running out of space in one place is usually caused by another.