Like in Twitter and Facebook, you can tag someone by putting an @ sign before their name, like
Hey, @lazy llama, maybe you can respond?
As you start to type after the @ sign, you will get a list of matching users. It's a good idea to select from this list to ensure you have got the correct spelling.
The person you tag will get an alert, with a link to the post.
You don't need to tag someone if you are replying to their post, as they will get an alert for that anyway, and generally people who have participated in a thread will be subscribed to it and will get an alert also.
To get alerted to tags, you need to have the appropriate preferences set: