Differences between @replies and mentions on Twitter are not always clear to every Twitter user. Therefore this short explanation.
@replies are tweets that are created by making a direct reply to a tweet (when clicking on “reply”), or tweets that start with the @ sign. Mentions are tweets that contain the @ sign followed by a username (“@username”). This means that every @reply will also be considered as a mention. You can find both types of tweets in the “mentions” section of your notifications.
But there is a difference between both types when it comes to the possible reach of your tweet as @replies will only appear in a user’s timeline if that user is following both the sender and the recipient of the @reply. So, if you are building a community and get your tweet spread, you can still use mentions, but just start your tweet with something else than the @ sign.
More information can be found on the Twitter support page.