<<< introduces a here string: the string after
<<< is passed as input to the command. This originates from rc (a Plan 9 shell), and is also present in zsh, ksh93 and bash.
<< introduces a here document: subsequent lines of the shell script are passed as input to the command, and the string after
<< is a terminator. Here documents work in all Bourne-style shells (Bourne, POSIX, ash, bash, ksh, zsh, …), C-style shells (csh, tcsh), and Plan 9 shells (rc, es).