I just came back from a 3 month trip through Europe on a Eurail pass, so here's my experiences with the train passes.
First off: This app makes the process simple. It does route-finding for the train system in Europe, and includes information whether reservations are required, recomended, amenities on the train, etc.
Reservations are rarely required. With the exception of trains into/out of major cities (Berlin, Paris, etc), you can simply board the trains and show the ticket checker the pass.
For the routes where a reservation is required it can be purchased online, or from the train station for a small fee (10-40 euros from my experience).
I would recommend not worrying about reservations until you're on the trip as things will come up, you'll find out about new events, and plans will likely change. I would plan out a general route with things you want to see/do, and adapt trip reservations/tickets as you go.
As for if you should buy a pass or buy individual tickets, it depends on how much you plan to travel. If you plan to go to 10 countries in 2 months, then I'd recommend one as routes between countries tend to be pricey. But if you're planning on staying in just a few countries buying individual tickets is likely cheaper.
There are also other modes of transportation worth checking out such as bus routes if price is a major concern.
How the pass works:
You get a pass that you carry around with you. If you have a set number of travel days you need to mark them on the pass before you ride each day, if you forget to fill in a date you'll possibly get fined. If it's unlimited then you can ride with no worries.
Regarding high speed trains:
Most routes without countries are normal trains, so unless you plan on doing very long distances at a time you probably won't take more than 1-2 high speed trains. When you do take them though, reservations are highly recommended (if not required). The seats are marked for which sections of the journey they are reserved for, and if it's a busy route you may end up moving often, or spending a large amount of the trip standing without a reservation. If you are on one of these trains and someone claims to of reserved the seat, make sure you take a look at their ticket to confirm it. On the other hand, if you're on a train and want a seat, you can tell a tourist looking person that you've reserved the seat. I booked my high speed train seats 1-5 days ahead, and I would recommend booking them at least 4-5 days ahead, more if there's a major event happening.
Note: For overnight trains reserving a seat/bed is usually required.
Information about reservations and how these passes work can be found here