It would take a very long time to learn all of the Unix commands, and frankly, that isn't necessary. Most Unix users have a subset of commands they use all the time, and that is how they learn them.
If you are asking about all commands (which are in fact, files) then the best way to learn how to use them is to use them frequently. Learning about "all" commands is not possible because new ones are created all the time, and it isn't necessary to learn all of them. Usually all it takes is a subset of what you need to use on a day to day basis.
Unix commands
Commands you use in a Unix based computer OS to achieve certain things. Similar to MS/DOS commands in Windows. Mostly used in computers running the Linux OS. unix command
The lp and lpr commands are the traditional commands used to print jobs on UNIX.
Because Linux evolved from UNIX, but Windows evolved from DOS.
Unix files do not rely on extensions, therefore there is no command to find them.
Man (or manual) pages
There is none. For starters, you have it backwards, DOS actually copied most of its commands from Unix (The rest came from CP/M.), which Linux is inspired by. Commands like "cd" and "dir" were Unix commands long before DOS even existed.
Most of MS-DOS' commands were based on those of Unix and CP/M. 'cd', 'dir', 'clear', and 'echo' are usually found in both. MS-DOS added it's own commands, however, and made some different from those of existing versions of Unix, and no one saw any reason to change the names of existing ones in Unix.
William Holliker has written: 'UNIX Shell commands quick reference' -- subject(s): UNIX (Computer file), UNIX Shells
Yes. Just about everything to do with Linux on the command line is case sensitive, including commands and file names.
The ones that are listed as part of a standard (such as Posix) will work in all UNIX systems (but not all systems, such as Windows command prompt). A lot of UNIX vendors add commands that only work in their version of UNIX, so they aren't as portable across systems as others.