When writing a comment, you can put an “@+username” to link to a specific user. He will then be informed via mail that he was mentioned in that specific comment. When working with Issuesyou can also assign the item to one or more users. They will also be informed. The @-feature is more of an addition if you want to point something out to a single user. Let’s say for example you create an Issue and assign Mark, Sarah and Justin to it. In the comment you might want to speak only to Mark so you will write something like “@Mark: please don’t forget the milk again – the others are counting on you”.
The #-feature works similar to the @-feature but instead link to a specific Issue. This allows you to mention an Issue in a comment without the need to copy and paste a long URL. Simply write “#+Issue ID”. What is an Issue ID you ask? Learn more about this pro-feature here.