DSSS is a way to make the radio spread the radio waves around. If you do the usual radio thing, concentrate a lot of energy in a narrow band, you get radio waves that go a long ways and interfere with each other. Spread them around a little, they get along better. So more users can crowd into a smaller area.
OFDM - A modulation scheme, as noted earlier. The signal that travels down a wire is not optimized for traveling on a radio wave. Radio signals need to change constantly to carry information. You have to convert the signal on the ethernet cable into something that can work in a radio environment. OFDM lets you put a lot of changes into a little time. So you get higher data rates.
There won't be an OFDM for dummies book. You have to understand many things and know some serious math to get it.
(for 802.11b) DSSS using 11-bit Barker Code to spread the 1 bit data. The chipping rate is 11 Mbps.The code used is:10110111000 (usually, 0 is replaced by -1).
(for 802.11g) OFDM is a modulation scheme, which has 52 subcarriers for data and pilot and another 12 subcarriers for DC. It is much more complex compared with DSSS.