I’ve just joined Twitter (after my nephew’s incessant urging) and I’m finding it interesting. I’m seeing messages with @s in them. What are these?

Hi there, and welcome to Twitter first of all. You’ll find it quite rewarding if you take the effort to get over the initial learning curve. I’m going to refer to the Twitter FAQs to answer your question, as they have a pretty succinct explanation of @replies:

An @reply is a public message sent from one person to another, distinguished from normal updates by the @username prefix. If a message begins with @username, we collect it as a reply. Reply publicly to any update on Twitter by using the @username format. Following is not necessary to reply to someone, and all of your replies are visible in the @username tab in your home page sidebar. (Tweets with @username elsewhere in the tweet are also collected in your sidebar tab; tweets starting with @username are replies, and tweets with @username elsewhere are considered mentions.) Please note that if your account is private, users who are not following you will not see your @replies or mentions.

When using a new platform, it’s important to get the terminology right, and the explaining defines what “mentions” and “replies” are and the difference between them. Now go ahead and tweet about your nephew, payday loans, your favourite car or whatever topic you wish!