So how does this election thing actually work?

So, the big day has arrived, and I have been trying to acquaint myself with some of the idiosyncrasies of the American electoral system, thanks to the BBC's US election 2016: All you need to know, which, as usual, manages to put it all into plain English for us outsiders.

First and foremost (and I say that because of the power that a president wields in the American system), Americans are voting for a president. The two front runners, as you might have heard, are a certain Hillary Clinton and Donald Trump, representing the Democratic and Republican parties respectively. The latest consolidated polls suggest that Clinton has about 48% of the popular vote, and Trump about 44% (this is after the FBI bombshell, and after the FBI bombshell was retracted).

But there are other candidates still running, some of whom might subtly influence the outcome (like independent candidate Ralph Nader did, to howls of anguish, in some previous elections). Gary Johnson, representing the Libertarian Party, is currently polling around 8%, and Jill Stein, representing the Green Party, has around 2% in recent polls. The other two, Darrell Castle (Constitution Party) and Evan McMullin (Independent), can safely be ignored.

The other important consideration to be aware of is that the proportion of the popular vote is not the be-all and end-all in this contest. Under the American electoral college system, the presidential vote is accumulated under "electors" in the electoral college for each state. The candidate with the most votes in each state becomes the candidate which that state supports for President. The number of electors in each state depends on the population of the state (e.g. California qualified for 55, New York 29, Wyoming 3, etc). So, within each state, the winner takes all, and the votes will therefore come in in batches over the evening's vote-counting. This is also why individual "swing states" - populous states whose political leaning is notoriously fickle and unpredictable, like Florida, Ohio, Colorado, North Carolina, Virginia, Nevada - take on such importance in American elections. There are 538 electors in total, so that the presidential candidate who reaches 270 becomes the President. But the electoral college system can skew the popular vote significantly, and it is quite possible for a candidate to win the presidency despite having a lower share of the popular vote. Perhaps the most egregious example of the electoral college effect occurred in 2000, when Al Gore and George W. Bush were neck and neck with only Florida votes to re-count (remember "hanging chads"?). All of Florida's 25 electoral college votes went to Mr. Bush, even though he won only 537 more popular votes across the whole state, and as a result Mr. Bush became president, despite receiving half a million fewer actual votes than Mr. Gore. And we all know how THAT presidency went....

But that's not all. As well as voting for a president, Americans are also voting for their local member in the House of Representatives, and 34 states are also voting for their Senators, as well as a handful of state Governors. The Senate and the House of Representatives together make up what is referred to as Congress. There are 435 seats in the House of Representatives, and the Republicans currently have a comfortable majority in the House (and have done since 2010, for various reasons including the vagaries of constituency boundaries and gerrymandering). It is expected that the Democrats may narrow that majority in this election, but are unlikely to pick up the extra 30 seats needed to establish a majority of their own. Only 34 (one third) of the 100 seats in the Senate are up for grabs during this vote, but the Democrats need a net gain of just 4 seats to win an overall majority, a target they see as eminently achievable.

As for which is more important, individual Senators may have more power than individual Representatives (just because there are fewer of them), but the separation of responsibilities in the US bicameral system means that the Senate controls things like, for example, making treaties, and appointing and ambassadors and Supreme Court judges, while the House has more control over the budget and how funds are used. Some responsibilities are shared (e.g. only the House of Representatives can impeach a government official, and only the Senate can convict them).

It is often suggested that the Senate.is the more important of the two chambers, but one thing is for sure: if either chamber is dominated by the other party, a president's ambitions suddenly become much more difficult to realize (take Barack Obama's experience as an example).

UPDATE
As mentioned above, Hillary Clinton narrowly won the popular vote nationwide, but lost the election by a substantial margin, all thanks to the electoral college system. And yes, many of the state's were very close, and the few percentage points of votes that went to third place no-hoper Gary Johnson could well have made all the difference in several states had they gone to Ms. Clinton instead (and could even have affected the outcome of the entire election).
As it is, Trump won handily, and the Republicans retained both chambers of Congress - the worst case scenario made real.