For kids between 5 pounds and 22 pounds, rear facing only.
Often called a pumpkin seat as most have a handle to lift the seat out and carry the child around. Also, most infant only seats have a base unit the carrier fits into.
You MUST read the owners manual for the child safety seat as well as your vehicles owner manual to make sure you are installing the seat in the vehicle correctly. If you have doubts that the seat is installed properly, call the child safety seat manufacturer and/or your automobile manufacturer.
Infant seats are popular with parents because most can be lifted out of a base unit that comes with the carrier and used to carry the child around. Manufacturers have also added a feature to some infant carriers that allow the carrier to be snapped into the manufacturers stroller or highchair.
If you are going to use the seat in more than one vehicle on a frequent basis, you may want to consider purchasing a second base unit to prevent the single base from having to be installed each time you need to move the seat to the other vehicle. This can assist in the base being put in correctly every time the seat is used.