The most powerful nation on earth is electing a new leader, with the impact felt across the globe.

When the US picks its president, it is not only choosing a head of state but a head of government and a commander-in-chief of the largest military on the planet.

The parties hold a series of primary elections in every state and overseas territory, starting in February, which determine who becomes the official presidential candidate.

The winner of each collects a number of "delegates" - party members with the power to vote for that candidate at the party conventions held in July, where candidates are formally confirmed.

The more state contests a candidate wins, the more delegates will be pledged to support them at the convention.

Democrat Hillary Clinton and Republican Donald Trump were the clear winners in 2016 and were officially nominated at their party's conventions in July.

They are two of the most unpopular candidates in modern American history.

At the conventions, the parties also officially unveiled their vice-presidential picks- Senator Tim Kaine of Virginia for Mrs Clinton, and Indiana Governor Mike Pence for the Republicans.

There has been a slew of controversies generated by Donald Trump, from the moment the New York businessman launched his campaign with the description of Mexican immigrants as "rapists and criminals". His candidacy has rarely gone a few weeks without sparking some uproar. He's waged wars of words with a judge, a Miss Universe, a Fox News anchor and the Muslim family of a fallen soldier. He's had to defend his refusal not to release his tax returns and the suggestion he has paid no federal income tax for 18 years, plus field questions surrounding his charitable foundation.

And Hillary Clinton has had her share of anxious moments too. The damage wrought by her private email arrangement to her reputation is thought to be significant, despite the FBI effectively closing the investigation in July and clearing Mrs Clinton of wrongdoing. Questions have been raised about the foreign donations to the Clinton Foundation. Mr Trump has also put the spotlight on the part she played in pushing back at the women who claimed to have affairs with her husband Bill. And Wikileaks has been revealing hacked emails that have laid bare some embarrassing conversations between members of her campaign team.

Most votes are being cast on Tuesday, on November the 8'th, although more than 40 million people took part in early or absentee voting.

The candidate with the most votes in each state becomes the candidate which that state supports for president.

It's all down to a system called the electoral college, a group of people who choose the winner - 538 of them, in fact. Just half of them - 270 - are needed to make a president.

But not all states are equal - California, for example, has more than 10 times the population of Connecticut, so they don't get an equal say.

Each state has certain number of these "electors" based on their population in the most recent census (it so happens that it's the same number of districts in a state, plus two senators).

When citizens vote for their preferred candidate, they're actually voting for the electors, some of which are pledged to one candidate, some for another.

But here's where it gets interesting. In almost every state (except Nebraska and Maine), the winner takes all - so the person who wins the most electors in New York, for example, will get all 29 of New York's electoral votes.

In the race to get to the magic number - 270 - it's the swing states that often matter most.

Actually, Donald Trump won the election and he started his work as the President of the powerful State on January the 20'th in 2017. But , many people organized any demonstrates against Trump's winning. However, Trump is going on his 45'th Presidential work and one interesting thing was that he refused from President's salary.