Ever start a tweet with a person's handle? For example, "@lif_au is my favourite site. You should all visit it!"? Unfortunately, if you structure your tweets like that, not all of your followers will see it. Gary Vaynerchuk says this is the top mistake everyone makes on Twitter.
When you start a tweet with someone's handle, the only people who will see that tweet are those who follow both you and the other person -- not all of your followers. This can be useful when you reply to someone and your tweet isn't all that useful to everyone. On the other hand, if you do want the tweet to be visible to all your followers, edit your tweet so it doesn't start with the @sign. Put a period before the handle or structure your tweet differently. That way you can rave about your favourite site or bash a company/person on Twitter for all to see.
The Number One Mistake Everybody Makes on Twitter [Slideshare]