If you're a Democrat, the delegates that count towards the nomination are being elected today. Attending a caucus is your only chance to have input on this. The primary ballot sitting on your counter will not count.

For Republicans, just less than half our your delegates will be chosen today. 

Think of it as an alternative form of primary. In a primary election, you picking which candidates will make it to the final election. A caucus serves that same function for major party presidential candidates. It's just a little different way of approaching it.

If you caucus, you will caucus either as a Democrat or a Republican (unless you identify with some other political party). In Washington, both parties hold their caucuses on the same day. But the two parties still meet separately. There is a caucus for each precinct.

I've honestly never been to a caucus before, but my understanding is that you will talk about politics and also vote on who you want for your party's presidential candidate. The results of the caucuses are used to determine how many of Washington's delegates will be pledged to each candidate for the party's national convention.

According to the information I was able to find about the Republican caucus system, the caucuses will select delegates to the county and/or state convention. The state convention will then select delegates to the national convention.

As emmittoconnell mentioned, the Democratic Party will determine pledged delegates solely through the caucus system. They will not use the results of the presidential primary election for determining delegates. The Republican Party will use the caucus and the primary system to determine their delegates.