UK election 2017- fixed term parliaments - what are they?

As a UK general election is anounced for June.A question people are asking is what is a fixed term parliament and how does it effect elections?

The 2011 Fixed Parliaments Act sets out the terms for early elections.
A simple explanation was offered by the Guardian
the House of Commons Library has also produced a briefing paper to the act.

the statement and reaction

Theresa Mays election statement - transcript from the Independent

Jeremy Corbyn twitter

Nicola Sturgeon SNP

Libdems

Green Party