The easiest way to look at it is, Tableau Online means that Tableau do everything for you (hosting, upgrades, bandwidth, etc..). Tableau Server means that you do it yourself (hence why the licensing is a bit cheaper).
Tableau Server cost
Tableau online cost
Please refer for pricing
2.If our data source sitting in cloud then go with tableau online but compare to server, online have limited data source connection option.
we can connect more data sources in tableau server.
3. Tableau server will be install in your organisation and online can managed by tableau