LOTW is a service provide by the American Radio Relay League or ARRL. It started in 2003. it allows a secure upload of your ham radio contacts to an online database. By matching your records with those turned in by the people you contacted the contact can be verified in minutes instead of months or even years which is how long it took with paper QSL cards.
There are other groups that offer similar services, like eQSL or club log, LOTW is the most popular.
Currently over 1 billion contacts have been recorded in LOTW.
LOTW records can qualify you for several award such as Worked all States (WAS) for making a contact in each of the 50 US states, or DXCC for making 100 contacts with different entities across the world. Most entities are countries, but some countries have several entities under their jurisdiction. For example besides the US mainland there is Alaska, Hawaii, the US virgin Islands, Puerto Rico, Guam, and American Samoa all of which count as separate entities.
Many users upload their data to LOTW world after every day they operate. Other many do so only occasionally and of course not all ham radio operators even use LOTW. I recently made a contact with a person in the Falkland Islands. This would have been another country toward my DXCC, but he does not use LOTW or even QSL Bureaus. I will at some point send him a QSL card, but I may or may not get one back from him and it will not be soon.