> is used to redirect output.
$ echo "hello" > file.txt
< is used to redirect input.
$ cat < file.txt
>> is used to append output to the end of the file.
$ echo "world!" >> file.txt
<< (called "here document") is a file literal or input stream literal.
$ cat << EOF >> file.txt
Here you can type whatever you want and it can be multi-line. It ends when you type EOF. (We used EOF in our example but you can use something else instead.)
<<< (called "here string") is the same as
but takes only one "word" (i.e., string).
$ cat <<< great! >> file.txt
Note that we could have used
$ cat <<< great! | tee file.txt instead of
$ cat <<< great! >> file.txt, but that would do something different.