"Hep-lock" and "saline-lock" are not devices, they are procedures, used to flush venous access devices, which include peripheral IVs, PICCs, and other types of central lines (Broviacs, etc.).
I've only worked peds. I gather with adults, people sometimes say "hep-lock" when they mean "saline lock". In peds we never did that, because there were some venous access devices that were always saline locked, and others that were always hep locked.
PIVs were always saline locked. This was done q 8 hours if the line was not infusing. The purpose is to keep the line patent when not being used, so that it will be available for use if/when you need it. Saying that a PIV was "saline locked" was another way of saying that it was not currently being used. At change of shift report, in addition to telling the oncoming nurse where the pt had PIVs, I would always tell them if it was infusing or not, and if not, at what time it was last saline locked. We worked 8 hours shifts, so PIVs would need to be flushed (i.e. saline locked) once a shift.
PICC lines in peds (at least the peds hospital where I used to work) are always hep-locked when not infusing. This is either done q12 hrs (for bigger pts with larger PICCs) or q 8 hrs (for littler pts). The procedure was to flush each lumen on the PICC with 3 ml of normal saline first, then 2 ml of a dilute heparin solution. The idea is that this small amount of heparin will prevent clots from blocking off the PICC, so again the purpose was to maintain patency of a venous access device not currently in use. And again, the last time the device was hep locked would be reported to the oncoming nurse.
I'm pretty sure that Broviacs were also hep-locked, but not certain, since I rarely worked with pts that had those (more common in oncology, which is not the type of unit I worked on).
The exact procedures for each type of device (mls of NS, mls of heparin, and frequency, i.e. q 8 or 12) was posted in our med room.
I hope that clears it up for you -- good luck with your new job!