You do know there are search engines on the internet? Ones that will tell you far faster than we can to start at https://confluence.atlassian.com/cloudkb/pros-and-cons-of-cloud-vs-server-691011844.html
No, you did not. You asked what the differences were.
That document is a useful guide to the main differences, so it answers the question you asked.
Your new question can also be found with search engines very easily. But you'll find that Server is a system you run yourself (or ask a provider to run for you) and Cloud is broadly the same software, but run by Atlassian for you.
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