The Johnson & Johnson vaccine uses a different method to prime the body to fight off Covid-19: a viral vector called Ad26. Viral vectors are common viruses that have been genetically altered so that they do not cause illness but can still cause the immune system to build up its defenses. The Pfizer-BioNTech and Moderna vaccines use messenger RNA to do that.
The Johnson & Johnson vaccine is rated as highly effective at preventing serious illness and death, as the Pfizer-BioNTech and Moderna vaccines are. It is also very effective at preventing milder illness, though a bit less so than those two. It appears to do well against the highly contagious B.1.351 variant, first identified in South Africa, that has given at least one other vaccine candidate trouble.
The Johnson & Johnson vaccine does not have to be stored at extremely low temperatures like the Pfizer-BioNTech vaccine. It can safely be kept in an ordinary refrigerator for three months, much longer than the Moderna vaccine, which spoils after a month if not kept frozen.
The Johnson & Johnson vaccine appears to be less prone than the Pfizer-BioNTech and Moderna vaccines to trigger the kinds of side effects that require monitoring after the injection, which may make it more suitable for use at drive-through vaccination sites. There have been reports that side effects tend to be felt more strongly after second doses, which the Johnson & Johnson vaccine does not require.