In the biology-related fields I'm familiar with, poster presentations are generally to one or a few people who are expected to interact with you, while slide shows are for larger numbers who are expected to be relatively passive. That means the biggest difference between a poster and slide presentation is that the former should be customized to the person you're talking to. Some people may be new to the field, and for them you may just explain what the question you're asking is, and the overall big-picture answer you found. The next person may be the person who defined the field, and for her you may simply point out that you used this particular variation of her technique rather than the original one she published in 1972.
The point being that it's typically a conversation, not a one-way speech. You have to feel out what the person does and doesn't understand, and try to focus on those, while not wasting time on the parts they know.
You might open with "Would you like me to explain, or did you want to read the poster yourself?". Often that leads to something (a detailed technical question, or a general question about the topic) that will tell you what kind of level to begin with. The key is to treat it like a dialog and to make sure you're responsive to the audience, even if it's body language rather than specific questions.