Timing Chain

a "true" roller has pins or "rollers" that well, roll (or spin, if you fancy that term) separate from the outer link, so they "roll" over the teeth of the timing gears. other, more common style is called a "fixed roller" or "fixed link".

you can get both in single or dual links.

the benefit is less friction and double "true" rollers are generally a little bit better made.

cam selection is entirely dependent on many factors of the engine itself as well as intended use. but the recommendation of going with a 340 cam probably indicates all of the specs be close or similar. of course you could start with the 340 cam as a baseline and then tailor aspects of it to your engine and combination-- but that's a whole 'nother ball of wax.