A tag applies to an individual commit, and is unique (i.e. you can't have the same tag on multiple commits); a named branch is applied to all commits in a branc

A tag applies to an individual commit, and is unique (i.e. you can't have the same tag on multiple commits); a named branch is applied to all commits in a branch and persists after merging. In the example mentioned by @krupan, the users tend to (and some team even require it) prefix each commit with the ticket name or similar identifier.