lpr is the off-line printer...
lp is just a frontend to the lpr. (that is in FreeBSD, maybe is the same in Sun Solaris), out put from man lp(1):
LP(1) FreeBSD General Commands Manual LP(1)
NAME
lp -- front-end to the print spooler
SYNOPSIS
lp [ -cs ] [ -o option ] [ -dprinter ] [ -n num ] [ name ... ]
DESCRIPTION
The lp utility is a front-end to the print spooler as required by the
IEEE Std 1003.2 (``POSIX.2'') specification. It effectively invokes
lpr(1) with the proper set of arguments.