Retweeting is of course a great compliment to those whose messages you forward, as it shows you have read their contribution, and think it worth sharing with your friends.
So yes Emily, you may retweet if you think your friends would be interested in what you have just read or seen, but there are things to remember:
If you simply retweet the message unchanged it will go to those of your friends who have not already seen it directly. These friends will be able to see that they have received it because you forwarded it, yet the original attribution will remain unchanged. This is a very good way to pass on the original snippet.
Should you wish to comment to your friends and there is enough space in the combined message, then you can prefix the original Tweet with your comment, the abbreviation ‘RT,’ and the original poster’s @tag. Do remember that the originator will also see your comment so be nice.
It would be really bad form to simply prefix the tweet with RT and the original @tag, as this means that all of your friends will receive the tweet again, even if they have already seen it, and you will be associated intrusively with it, but without having added any value. This would reflect a lack of understanding and consideration on your part, and should therefore be avoided.