Where?

Level 1: Areas of the body which can produce orgasms when stimulated.  In men, this is the penis, especially the head and rim, or frenulum.  In women, this is usually the clitoris and vagina.  

Level 2: Areas of the body which will intensify sensation and increase sexual excitement when touched.  In men, this means the testicles, inner thighs and the area between the base of the scrotum and the anus.  Nipples and breasts are also secondary erogenous zones, some women even find that stimulation here can lead to orgasm.

Level 3: This can refer to the whole skin surface.  Tertiary erogenous zones are areas of the body which can produce an erection in men or vaginal lubrification in women, and a light touch on any area of the body can do this.  It all depends on how excited you are already and on your own personal physical history – a certain touch in a certain place may bring back sexy memories which turn you on and a body which has always been cuddled and stroked will be much more responsive to contact.  An injured or abused body may be less responsive, or even react in a negative way to caresses.