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. (Masters sets never enter Standard.1)
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 releases sets into Standard has changed twice. For most of Magic's history, each 12-month period saw one new core set and one new three-set block.
In 2014, the set release pattern was changed such that each 12-month period saw instead two two-set blocks, with core sets abandoned.2
In 2017, the set release pattern was changed again such that each 12-month period sees once again one core set and three non-core sets, but with the concept of blocks completely removed. This change will start to come into effect in Q2 2018.3