Regarding tops... There's pretty much two and half reasons I can think of to have a top.

1. It keeps jumpers in the tank.
2. It helps keep heat in the tank. I know my house can get pretty cool in the winter, and it's nice to have the help of the top to keep in the heat.

And the "half" reason, it just helps keep dust and stuff out of the tank. Not a biggie, but it's something.

You mentioned "lots of fish". Try to follow the 1"/gallon rule. For each gallon of water in the tank, you can have 1" of fish. Being a 65G, you'll have plenty of room for fish, but it is easy to overstock - even a large tank like that. Also, add your fish slowly so as to not spike levels in the tank.

Finally, for a good list of abbreviations, see this link:

Common fishkeeping abbreviations