There are 3 built-in file descriptors: stdin, stdout and stderr (std=standard).
Basic
0/1/2 references stdin/stdout/stderr
>/2> redirects stdout/stderr, where > is taken as 1>
&1/&2 references stdout/stderr
&> redirects everything
> creates or overwrites a file
>> creates or appends to a file
< reads from a file, where < is taken as 0<
Note: You should be pretty sure of what a command is doing if you are going to wipe it's output.
Redirecting Examples
stdout to file: ls -l > ls-l.txt
stderr to file: grep da * 2> grep-errors.txt
one to another: grep da * 1>&2
everything: do_stuff &> /dev/null
combined: command < input-file > output-file, reads from input-file and writes stdout to output-file
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