The short answers to your questions are :
**Does it vary according to the type of STD? ** yes.
** The type of sex?** Yes, that too.
If you have sex with someone with an STD is it safe to say you just caught it?
I wouldn’t use the word “safe” there. But no, having sex with someone with any STD doesn’t mean you now have it.
Why are condoms effective in offering some protection? Your dick is covered. But the rest of the genitals are exposed to body secretions.
The reason why I understand you asking in a forum of strangers, instead of just looking it up online, is because there are people who will tell you that anyone with an STD will cause you to get it too, if you fail to throw rocks at them from a distance and scream curses as they run away down the street.
A few general things:
Some STD’s are easier to catch than others. Some are harder to know the person has. Some have such low level symptoms in SOME people, that they don’t even know they have them, and then they pass them on to someone else.
The main reason why some people say not to trust a condom, isn’t because a condom wont work per se, it’s because people who are having a really good time, often fail to use a condom PROPERLY. This is because using one properly, will limit your fun.
Some STD’s are only transmissible SOMETIMES. Some people are only susceptible to some STD’s SOMETIMES.
Some STD tests are more reliable than others. Some give false positives. Some give false negatives. And you have to get a specific test for the STD you are concerned about…if you don’t specify, you wont know. Some STD’s are actually not yet understood well enough to say anything definite about them, so we get wide cautionary messages. Some STD’s have cures, and some don’t.
Some people who give official advice, want to avoid being sued, so they will tell you that you can catch an STD from a sneeze or a handshake. I heard a show on the radio (a long while ago now) where they were talking about the risk of getting pregnant, and they advised young people that they could get pregnant even if they never took their clothes off. They wouldn’t spell out the truth, due to censorship and the desire to discourage all young people from having sex.
Hope that helps, but I doubt it will, even though it’s accurate.