A player must purchase an unlock code from the publisher to unlock the full video game legally after entering the code in the game menu's unlock input field. Authorized resellers sell video game unlock code cards for downloadable video games. These codes are internal; the player simply downloads the game.
Valve's game platform, Steam, redirects the player from within the demo version of the video game to the Steam store after the player has agreed to consider buying the game. When the player pays for the game, Steam downloads the necessary files to the player's computer, which unlocks the full version of the video game.
Video game demo versions attempt to persuade the player to purchase the game by demonstrating the gameplay while limiting the amount of content available to the player. After the player completes all that the demo version has to offer, the demo version prompts the player to purchase the full version of the game. This advertising method is effective for the video game's publisher, and it allows the player to see if the game is right for him.
A demo is a sales pitch for a product to a potential buyer. Video game demos became popular in the 1990s and were known as shareware.