I'll try explaining how the primaries work to the best of my knowledge, since I've done a little research.
When it comes to primaries, you have the standard primaries and then the caucuses. The normal primaries just mean you go to the polling place you're assigned to (based on the district in which you have permanent residence) and vote for who you want in your state. But for states that have caucuses (Which is in the minority of states), people have to physically show up and get counted by hand, and to their specific assigned locations. Caucuses are more of a pain since you have to stay there a while and it can interrupt work schedules and the like, and perhaps it can be more overwhelming with the amounts of people.
As for turnout, I think the general trend has always been that it's not usually that high.
This article says in 2012 only about 15% of eligible citizens went to vote. But you have to consider that there are times where everyone is expecting a certain candidate to get nominated so they probably don't even bother. The
actual election day turnouts are better, ranging from high 50s to mid 70s. In either case, turnout amongst younger voters tend to be really low. Whether it's apathy for voting, or they just don't like anyone that's running, I wouldn't know. Although Iowa last night had a higher than usual turnout among youth- likely because of Sanders' movement.
Finally, yes, primaries/caucuses do affect the general election. Each state gives a candidate a particular amount of delegates in proportion to how many people voted for that candidate. AFAIK, the delegates in turn are supposed to basically represent people's interests but also decide who each party (Democrat/Republican) should nominate for their presidential candidate. You need a certain amount of delegates to be nominated. That way, you narrow down the possible Republican candidates (Trump, Cruz, Rubio, etc.) to a single person. Democrats do the same, and then on election day each states votes one or the other.
The real issue comes with superdelegates, who are delegates that can basically represent whoever they want regardless of what states vote for. So even if someone wins by a tiny majority based on state delegates, the superdelegates can swing the vote so their opponent wins.
Hopefully this clears some things up for you. If anyone has something to add or correct, please say so.
Also, I think for Bernie, one of the most appealing things about him is that he's not backed by the industry, and he has had the same opinions for all the years he's served in Congress. Can he deliver on all his promises if he's elected? Unlikely, especially considering the Republican majority in Congress would try to block any major changes. They did the same with some of Obama's policies. Are all of his policies amazing and going to save America? Probably not. But many see choosing Bernie as making a kind of statement, one that says "We're tired of the same old politicians and how they always look out for the bottom line," and even if he hasn't set out everything he wanted to fulfill, that having him is a better alternative to Clinton (with policies that change based on what's popular) or Trump (because of his fear-mongering and all the other dramatic things you see him on TV for).