Magic tournaments are run at three different Rules Enforcement Levels (REL); Regular, Competitive an Professional.
Most local tournaments and some side events at larger events are run at REG, or Regular, REL. Even if you’ve been going to events for years, these events are considered to be introductory tournaments and the focus is on fun and friendly play. In the last few years the policy and rules that govern REG events has separated from the other RELs and REg event now have their own document, Judging at Regular. This two-page document is designed to cover most difficulties and issues that would come up at these events. Prereleases and Friday Night Magic are two of the more common Regular REL events that you will come across.
Competitive events are for those events with a larger prizes on the line and the need for harsher penalties to both encourage players to stay on the straight and narrow and to prevent players from accidentally gaining an advantage. Competitive policy is covered in the Magic Infraction Procedure Guide. Examples of competitive events would be Grand Prix Trials, Pro Tour Qualifiers and day one of Grand Prix.
Large premier invitation only events that are run directly from Wizards of the Coast or are contracted out to experienced tournament organizers, they have significant prizes and use Professional REL. The penalties at Professional are almost identical to those that are used at Competitive events, but the staff at these events is less lenient and do expect players to play on a rather high technical level. Pro Tours, Worlds and day two of Grand Prix are all run with Profession al REL.