Wizards of the Coast sells the Player's Handbook , which has all of the rules and options for the game at its release time. They also publish a free pared-down version called the SRD or "System Reference Document". This can be freely distributed, and adventured can be built from it or third-party products released without purchasing a license from WotC. WotC is very careful about their property and who they allow to license and for how much. So most sites and products use the SRD. The SRD has only one background, one feat, one archetype for each class and a reduced spell and monster list. The 5e Compendium on Roll20 (and all legal SRD sites) contains only this information. If you find other sources on the internet that have (for instance) the Storm Cleric or the Warcaster feat, that is almost certainly an illegal site that has either escaped the notice of the lawful IP holder (WotC), or is outside the jurisdiction of International copyright law (i.e. a pirate site). There are no free sources of this information. Roll20 sells licensed copies of the information in the Monster Manual, all formatted and ready for play. At this point in time, they do not have a licensed version of the information available in the Player's Handbook. The purpose of releasing the information this way is to allow people to learn the game (and thus want to buy the full rule set), and to allow for the growth of the hobby by allowing third parties (which are usually small groups or even individuals) to produce content without the prohibitive cost of a full license. This way of presenting their material was introduced with the 3rd edition version of the rules. Don't worry, this question comes up quite often.
