There are two distinct but related kind of herpes-viruses: type 1 and type 2.
Type 1 causes a cold sore, also called a fever blister, around the mouth or/and on the lips. But this can also appear elsewhere on the body.
Type 2 is a STD, which causes blisters on the genitals.
When you kiss someone who has a cold sore you can be infected by the type 1 virus. You can get symptoms after two days, but it can also take three weeks before you notice.
This type 1 virus can be transmitted to the genitals. So if your lover has a cold sore and gives you oral sex you run the risk of getting infected. It also works the other way round. If your lover has genital herpes you can get a cold sore on your lips if you give oral sex.
Genital herpes are in 80-90% of the cases caused by type 2 and in 10-20% of the cases by type 1.