Condoms protect against HIV and also reduce the risk of contracting sexually transmitted infections such as syphilis, gonorrhea, hepatitis B and C. (You should be vaccinated against hepatitis B if you frequently have sex with different partners).
Please note: condoms will never provide 100% protection and are only reliable when used correctly. The following applies to both, size and usage: practice makes perfect!
When giving a blow job without a condom, there is a risk of an HIV infection when cum gets into your mouth. However, this risk is much lower compared to having anal intercourse, because the mucous membrane of the mouth is more resistant to HIV than other mucous membranes in the body.
Precum also contains the virus albeit in lower concentrations. This is not sufficient for a risk of infection through oral sex.
You can find more about protection under the heading "Safer Sex".
For more information click on the following terms: