A clamshell is a one-piece container consisting of two halves joined by a hinge area which allows the structure to come together to close. Clamshells can be made to be reusable and reclosable  or can be sealed securely.
Clamshell containers can be made of a variety of materials. Plastics such as polystyrene, polyester, PVC, foam sheets, etc. The material can be made by thermoforming or can be injection molded into the desired shapes. A single piece of material is used for the top and bottom with a "living hinge" that is integral to the material, rather than added separately. Clamshell packaging works great for packages that need to hold heavy products, or items like electronics that are a target for theft.
Folding cartons made of paperboard or molded pulp can also be of a clamshell shape.  It can also be made of cellulose fiber such as sugarcane-bagasse, wheatstraw, wood pulp, etc.
Clamshells can use a variety of means of closing or sealing. Some have self-locking tabs, snaps, or have a friction fit. Others use adhesive, pressure-sensitive tape, labels, staples, or are heat-sealed.
Many clamshell containers are easy to open, and reuse, by consumers. When plastic clamshell containers are securely heat sealed, they are tamper resistant and deter package pilferage.
These security packages are intentionally difficult to open, sometimes requiring customers to use scissors or a knife to open. Scissors have been demonstrated as effective at opening packaging of this type.
Difficulty opening such packaging can be frustrating to the point of wrap rage. Some people injure themselves trying to open security packaging which in the United Kingdom has been cited as the most frustrating to open.