To expound a bit on Drag Bucket's explanation (which is quite adequate for most purposes):
IMU -- Inertial Measurement Unit
INS -- Inertial navigation System
IRS -- Inertial Reference System
GPS -- Global Positioning System
RNAV -- Area Navigation System
LNAV -- Lateral Navigation
VNAV -- Vertical Navigation
FMS -- Flight Management System
The various Inertial* stuff are various names appended by various vendors to various means to navigate via a "stable internal reference" around the world. The "system" may consist of 1 or more (usually no more than 3) IMUs, which may be the "old" mechanical gyroscopic systems or the "new" RLG (Ring Laser Gyro) systems.
RNAV is any system that allows you to navigate directly from any randomly-selected "waypoint" to another. The "old" RNAV systems used VOR and DME to define the waypoints; the newer INS and newer yet GPS systems use Latitude and Longitude.
The FMS is a system of computers that integrates the position information available from the INS/RNAV/GPS system(s) with the performance information from the aircraft. It provides, among other things, the functions we call VNAV and RNAV that attempt to best integrate the airplane's performance characteristics with where we want to go in 3 dimensions.
All the alphabet soup plays in the equations, but the equations are different for each airplane...