THP is a browser game where you play as a hacker. You start off with crappy hardware and software, and so it is your job to make your way up in the world. Now this is where the game gets a bit interesting. At the very beginning of the game you could theoretically get the best software ever created for nothing. To do this you just have to go onto, or hack some computer with the software and download it off of there. What prevents a person from doing this is the fact that every piece of software costs a certain amount of CPU cycles, memory, and Bandwidth to use, and at the start of the game your computer can't handle the software, as well as the fact that to download software of extremely high ranking(version) you must wait literally days in real world time.
So this structure forces the player to take missions to build up their income and therefor build up there computer's hardware. As the player increases their hardware he or she builds up their software library. Which in turn allows them to take on more difficult missions which award the player more money on their successful completion. All this is a self feeding loop that keeps the player going till he or she gets bored.
At this point you may be asking yourself why a player would subject themselves to what sounds like boring, monotonous work for nothing? This is where the game get especially clever; there is a high-score bored that lists the top 10 best hackers in the game. For each position on the bored that player will receive some money. So for example the 10th place player will get 10 dollars, but the 1st place player will get 60 dollars. This causes the players of the game to have real motivation for their work. But then on top of this there is the fact that for these player's names to be written to the high-score board and for them to receive their money they must activate an in-game virus on 10 different servers, that takes 24 hours to complete its run. In this time the virus can be stopped by an anti-virus also applied by players.
So this generates a real rivalry between the top 10 players wanting to activate the virus to get their money, and the other players who are not in the top 10. This generates a combat between players that ranges from deleting another player's software, to installing a virus on their computer to steal their money.
The result of this is an addicting game, much like gambling. Accept for the fact that the players themselves control their own destiny, and there is always the chance that you may be the one to rise to the top and make your money.