CPR stands for cardiopulmonary resuscitation and certification in performing it is often required for those pursuing work in the fitness or health and medical field. While the Internet has created many opportunities for home study, CPR is one of the things that still requires hands-on training.
The American Heart Association (one of the main CPR certifying bodies in the United States) does offer an online course, in which you can learn CPR theory through self-paced modules or videos. Once you feel you have mastered the information, you will need to call the AHA and make an appointment to meet with an instructor, who will teach you the hands-on procedure and then test you to make sure you learned the necessary skills. Once all three steps have been completed, you will receive a certification card valid for two years.
Some companies, like the CPR Prompt Home Learning System, offer a complete program you can use to learn CPR in the comfort of your own home. This includes a video course, a printed guide and a set of mannequins (adult and child). The mannequins are programmed to provide feedback on whether or not you are providing the right pressure on the right place, and the course also includes lots of accessories, such as illustrations and audio tips to help make the learning process easier. However, there is no official certification attached to this or any other home-only CPR program and you won't be recognized as certified by any major organization or body.
Companies that promise recognized, complete online CPR certification are basically scams. The regulating bodies for CPR certification, including the American Heart Association and the American Red Cross, do not recognize any online certification as legal. While studying at home can teach you the needed skills to perform CPR, you won't be able to use the certificate you obtain to enroll in any type of certification course that requires CPR certification, such as personal training, nursing, home health aide, etc.
Many companies that advertise CPR certifications online are actually selling first aid courses. Because they don't focus exclusively on CPR, they are allowed by law to offer a certificate of completion, which is still not recognized by the certifying bodies mentioned above.
When looking at online CPR certifications, some things to keep in mind to verify whether the certification is legitimate or not, are: a combination of distance plus hands-on instruction; a final test that must be passed in front of an instructor; the seal of the American Heart Association or Red Cross validating the certification; a CPR card/certification that must be renewed (lifetime certifications don't exist).