Depending on the time of year, Standard can contain anywhere from one to two core sets and from four to six block sets. Confused? Read on.
Wizards of the Coast releases blocks (a block is a group of three sets) and core sets alternatingly. Every time the first set in a block gets released, the oldest block in Standard and the oldest core set in Standard simultaneously drop out.
This means that Standard can only ever contain two blocks at once, even if one of those blocks has only had one of its sets released.
This also means that core sets spend less time in Standard than any other set, as they are the last to be released but they don't drop out any later than their block set brethren.
Note: All previous prints of a card currently in Standard are also considered to be in Standard. For example, if you own an Eighth Edition Naturalize, you may use that print in a Standard deck.
In late 2015, set release patterns will change in the following ways:
What this means for the Standard format:
In late 2016, the last of the sets released using the current pattern will drop out.
For more information, see the very well written official announcement article, which explains the changes and their intended effects.