They fundamentally do the same thing in that they will alert you (using the button at the top of the forum) when a new post has been made to one of the topics you are subscribed to or are watching.

However, subscriptions do more in that they will (1) also send you an email notification and (2) if the forum admin has allowed it – send emails automatically to a topic you have posted to that receives a new post. In the forthcoming V5.1 subscriptions have also been extended to include digests and more.

Watches – on the other hand – are more passive and only announce a new post using the UI button.

Thanks for explaination

You’re welcome