A clamshell or clamshell mobile is a mobile phone form factor where the phone is divided into two equal halves with a hinge connecting the halves, enabling users to basically fold the phone in half to close when not in use. This type of mobile phone usually has the buttons on the bottom half and the display and speaker on the top half. Some clamshell mobiles may also have a small display on the outside, so when the phone is folded you can view connection details, incoming calls and the date and time without opening the phone.
Clamshell phones are more frequently called a flip phone. Flip phone is used in everyday language as a generic term, however Motorola owns the trademark on the term flip phone.
See also "The Difference Between a Cell Phone, Smartphone and PDA" in the "Did You Know..." section of Webopedia.