ERP is an acronym for "extended rate PHY", as described in Clause 19.
OFDM is an acronym for "orthogonal frequency division multiplexing." This is the name of the PHY described in Clause 17, AND the name of a modulation technique first used in the OFDM PHY and later used in the ERP. (Saying "ERP PHY" is redundant but sometimes necessary to carry a point - see below.) When the OFDM modulation is used by an ERP device, the ERP modulation is called ERP-OFDM.
The OFDM modulation is the primary modulation technique for HT PHY. This swiss army knife of 802.11 goodness when fully developed will interoperate with other HT PHYs (2.4 and 5 GHz), ERP PHYs (2.4 GHz), OFDM PHYs (5 GHz), HR/DSSS PHYs (2.4 GHz), and DSSS PHYs (2.4 GHz).
I hope this helps. Can you add your location to your forum profile? Thanks. /criss