|A poultice of red clover on a rash on my own leg.|
You might think poultices aren't used very often in the modern world, perhaps because you haven't heard the word used in common conversation, but you'd be wrong. If you've ever put a little bit of ointment, perhaps some Polysporin, on a bandage and applied it to a wound, you yourself have used a poultice. They're also used frequently in hospitals for a variety of purposes, though they are more likely to call them applications.
Simply put, a poultice is just a sterile cloth (such as a bandage or even a sterile piece of cheesecloth) that is used to keep some for of medicine in place. This medicine might be a paste, it might be an ointment, or it might even be actual loose herbs. In general, if you do use loose herbs, you'd place a single layer of bandage or cheesecloth on the wound first, then place the loose herbs, then bandage the wound. This practice is sometimes used with pastes and ointments as well, depending on the ingredients. The poultice is often heated, but be careful of applying an overheated poultice to bare flesh. Burns can and have resulted.
Back before we knew much about infections and such, a piece of bread or other similar food product might have been used instead of the sterile cloth. I distinctly remember my grandmother applying a poultice of mustard and a few other ingredient to a piece of bread and strapping it to my spider bite. It worked and I didn't get an infection, but today it would be better to use sterile cloth. We don't always have to be stuck in the past, after all.