Resort fees are a fairly new creation. Hotels advertise low rates to hook you in then charge an additional "resort fee" of anything from £10 to £30 per night on top for things like gym access, use of the pool and other facilities, and wifi, thus bumping up the overall cost. You'll mostly find these at resorts frequented by American tourists (especially in places like the Caribbean, Mexico, and the resort fee heartland of Las Vegas). They are usually not optional, though some people have managed to talk their way out of paying. Read the small print to make sure that wonderful deal isn't hiding a dirty little secret!
Also, remember to look into whether you have to pay anything at the airport. Some countries charge entry fees and departure tax, which can vary wildly. Sometimes it will be included in your airfare, but it's worth checking, as you'll often need to pay it in cash (in the local currency).
Image Source: POPSUGAR Photography / Mark Popovich