Skimming is the theft of ATM or credit card information during the course of what appears to be an otherwise legal transaction. ATM skimmers are designed, for example, to acquire the ATM card number and then, through a variety of different devices also acquire the PIN. This allows the thief to collect the data and then use it to get access to the account.
KrebsOnSecurity (via Boing Boing) had a very interesting example of one such skimming device (see picture) with links to pictures of other such devices. A casual search of the internet yielded many, many other examples (including this YouTube video). Lifehacker also linked to a very good PDF by an Australian firm with some detailed info on both the skimmer and the PIN capturing devices.
This type of fraud has been around for some time now and the tricks used by the bad guys continue to get more sophisticated. Despite this, it seems that many people are not aware of the risks. It is worth taking a look at Krebs and the YouTube video simply to be armed with a little bit of info.