General question about batch\ help files

Hi all, when ever I'm looking a batch file to do something or details on how to run dsquery for example I'm always greeted with something that looks like this:

dsquery user [{<StartNode> | forestroot | domainroot}] [-o {dn | rdn | upn | samid}] [-scope {subtree | onelevel | base}] [-name <Name>] [-desc <Description>] [-upn <UPN>] [-samid <SAMName>] [-inactive <NumberOfWeeks>] [-stalepwd <NumberOfDays>] [-disabled] [{-s <Server> | -d <Domain>}] [-u <UserName>] [-p {<Password> | *}] [-q] [-r] [-gc] [-limit <NumberOfObjects>] [{-uc | -uco | -uci}]

Here's my question. I'm not asking about dsquery just asking about what some of teh syntax means above.
1. The top line starts with a [ followed by {  and then some stuff witch a couple of pipe commands and the ends with a } followed by ]. What is this telling me, to use the brackets not to use them i really have no idea? As you look down the example you will see more of them. "2nd, i often see reference to stuff inside <> as above, what's that about. If anyone can give me a break down of the above (not what dsquery does) just what all the brackets and stuff mean I'd really appreciate it.
Thanks for looking