Latest: At the Democratic National Convention in Philadelphia, Hillary Clinton made history as the first woman to be nominated for US president by a major political party.

During her speech on opening night, First Lady Michelle Obama spoke about the importance of black and female role models in the White House and why America should vote for Hillary Clinton.

As the race to the White House continues ahead of the presidential vote in November, here’s a guide to how US elections work.

1. Why do US presidential elections take so long?

Early in the year of the presidential election, Iowa holds the first caucus and New Hampshire the first primary.

These two polls follow months of campaigning, while there are a further five months or so between the firing of the starting gun in Iowa and the point at which the Republicans and Democrats have officially selected their candidates at their political conventions, being held this year in Cleveland and Philadelphia, respectively.

Once that point is reached, the rest of the summer and autumn is spent campaigning before the election itself on 8 November.

Long campaigns go some way to explaining the cost of US elections.

In the 2012 presidential campaign, Mitt Romney is estimated to have spent the equivalent of $13 per vote, as did Hillary Clinton in her 2008 bid to win the Democratic nomination.

Image: Washington Post

2. What’s the difference between a caucus and a primary?

Outside the United States, most people would be hard pressed to tell the difference, but a primary is run by the state while the political parties run caucuses.

A primary is more like a traditional election with ballots cast at polling stations and a broad range of voters taking part.

Caucuses take place in churches, schools and even people’s homes. The meetings will often discuss candidates before voting and may deal with other political party business. Because of this, caucuses tend to involve more ideologically committed party members and far fewer people turn out.

Both parties have a set number of delegates – party officials in each state who ultimately get to choose the official candidate at the party's national convention later in the year. Either caucuses or primaries are used to decide how many delegates each person hoping to secure the nomination gets. The person with the most delegates will win the nomination.

So are Democratic and Republican caucuses the same? No.

Republican caucuses involve a straightforward ballot for a preferred candidate. The votes are counted by the caucus chair and the winner is announced. There is no minimum threshold a candidate must reach. The winner is simply the candidate with the most votes in each caucus and delegates are allocated on this basis.

The Democrats make things a little more complicated. Caucus-goers physically stand in different areas of the room to show their support for a particular candidate. This is called aligning. During a 30-minute alignment period, people work the room to try to convince others to support their candidate.

Supporters of candidates who do not meet the "viable" threshold, usually about 15% of those present, can either join another candidate’s group or try and encourage others to join them. The number of delegates each viable candidate gets will depend on the strength of their support.

3. Do the caucuses usually predict the eventual nominee?

For the Democrats, yes. For the Republicans, no.

Most recently, Barack Obama’s victory over Hillary Clinton in the Iowa caucus in 2008 did much to propel him to the White House.

In 2004, John Kerry beat Howard Dean and went on to win the Democratic nomination.

In fact, since 1976, Iowa Democrats have got it wrong only twice: in 1992 when Iowa native Tom Harkin won the caucuses and in 1988 when Dick Gephardt from neighbouring Missouri won.

Since 1980, Iowa Republicans have accurately predicted the official nominee only twice: Bob Dole in 1996 and George W. Bush in 2000. In every other caucus, Iowa Republicans chose candidates that did not go on to get enough support to secure the nomination.

One reason for this is that Iowa Republicans are seen as more conservative than the national average with a tendency to go for more socially conservative candidates.

4. When are the presidential candidates officially nominated?

Following the results of state primaries and caucuses, both parties formally nominate their candidates for president and vice president at their conventions in the summer.

These four-day media extravaganzas - complete with eye-catching hats - are a chance for the nominees to sell themselves to voters and for the party to set out its policy platform.

Conventions generally consist of a string of speeches by politicians, ending in a lengthy address by the nominee. But, as this week's Republican gathering in Cleveland shows, there's enough drama in between to keep audiences entertained.

And this is before the real election campaigns have even started. After the summer, the two candidates travel across the country to audition for the role of president.

5. What are the main challenges the winner faces?

After years of battling to secure their party’s nomination and then the popular vote, whoever wins the presidential election in November will find that the really hard work starts when they arrive in the Oval Office.

Income inequality, lack of infrastructure investment and rising health care costs are all pressing issues in the US economy.

Keeping the country competitive is also a major challenge, with China and other emerging economies continuing to put pressure on the more established nations.

The World Economic Forum’s latest Global Competitiveness Report placed the US at number 3 in the world ranking, but maintaining that position is going to be tough.

Sustaining robust economic growth in an environment of slowing global growth will be a significant challenge, whoever finally makes it to the White House.