Update():- if you are sure that the session does not contains an already persistent instance with the same identifier then use update to save the data in hibernate

Merge():-if you want to save your modificatiions at any time with out knowing abot the state of an session then use merge() in hibernate.

Merge:Suppose if we take two words like.. one is milk two is van
if we merge this two words it forms milkvan.

Update:Means if we have know something about JAVA language there are soo many new things are being added to the java language if we know all or some portion of new things with existing things are taken as update.

Use update() if you are sure that the session does not contain an already persistent instance with the same identifier and merge() if you want to merge your modifications at any time without consideration of the state of the session. In other words update() is usually the first method you would call in a fresh session ensuring that reattachment of your detached instances is the first operation that is executed.