The voting system used to elect MPs to the UK Parliament is called First Past the Post (FPTP).
For the purpose of the election, the UK is divided into 650 areas or constituencies (seats) and at the election the candidate with the most votes becomes the MP.
In addition to voting for a local MP, voters are also taking part in choosing a government. Under the FPTP system the party with the most MPs becomes the government. In 2015, the Conservatives had more MPs than all the other parties put together so they became the government.
In 2010, unusually, no one party won a majority of the MPs (had more MPs than all the other parties put together) so the Conservative Party (the largest party after the election) invited the Liberal Democrat Party to share power in a coalition government.