In Git, 'merge' and 'rebase' are two different ways of integrating changes from one branch into another. 'Merge' takes the contents of a source branch and integrates it with the target branch. When a merge happens, it always results in a new commit because it creates a new snapshot that reflects the combined changes from the merged branches. This maintains a history of all commits from all branches chronologically. 'Rebase' on the other hand, moves or combines a series of commits to a new base commit. Essentially, it's taking the changes made in the source branch and reapplying them on the target branch. This helps to create a linear, easier to follow project history. When to use which? 'Merge' is a good option when you want to combine code from two different branches and your team needs to see the history completely and chronologically as it is. But if the commit history is not that important and you want a cleaner, linear history then 'Rebase' is the best option. Remember: With Rebase, it's possible to change commit history, which is good for making a more organized and readable history. However, it can also be dangerous if not used carefully. In a nutshell, both 'merge' and 'rebase' have their own advantages and it's up to you and your team's preferences to decide when to use which.