High risk and low risk refer to the chances of acquiring an infection given a specific set of circumstances. Usually doctors and epidemiologists look at risk per act numbers/estimates. So highest risk is blood transfusion
partner, then unprotected insertive anal sex with a known HIV partner, then so and so for vaginal.
If the status of partner is unknown, the risk is considered to be less given the prevalence of HIV infection in a population.
Low risk activities are those where it has not been proven to transmit HIV infection, such as getting a blowjob or protected sex with a condom. We can't say no risk because we're covering our asses.
Actually, the risk from High to Low is as follows:
Blood transfusion
IV drug use
Receptive anal
Needle stick
Receptive vaginal
Insertive anal
Insertive vaginal
Receptive oral
Insertive oral
Go to http://www.aegis.org/pubs/mmwr/2005/RR5402-1.html
Table 1.