Standard is a rotating group of Magic: The Gathering sets. Most sets enter Standard when they're released and drop out about eighteen months later.
A deck which contains only Standard cards is called a Standard deck.
If you're coming back to Magic after a break, note that the way Wizards organizes blocks has recently changed. Previously, each 12-month period saw one new core set and one new three-set block; today, each 12-month period sees instead two new two-set blocks (core sets are gone).1
As of October 2016, rotation size and frequency are the same as they were pre-2014.2