Especially for the price, the 100 is a great headphone. You have probably seen AKGs if you've ever set foot inside a recording studio. Their upper end models are comfortable and flat, meaning that they color the sound only minimally. The 100s are not very heavy and even have a real leather headband. A fine starter overall.
Open construction means that the speakers have open backs.
The sound escapes away from your ears a good bit with this type of design. I like this design and love Beyerdynamic headphones. The sound is flat, full, and clear as hell. They are also very, very comfortable. You may have to search around for Beyerdynamic products, but the search will be worth it.