Research

Most medical schools want to see some research experience. Generally speaking, the more the better.

It really depends on the school you apply to, though. Some medical schools don't care at all whether you have done research or not. However, most medical schools prefer to see some research experience on your application and yet other schools absolutely require it and won't consider you if you have done little or no research. Generally speaking, some minimal exposure such as working in a research lab for a semester is okay for most schools.

However, as already mentioned, some schools want to see major research involvement, perhaps that you have done your very own project, and may prefer published work. Especially if applying to research "powerhouses" and top medical schools, this will probably be necessary.

See what successful applicants did for Research.

Some professors help undergraduate students set up their very own research projects that lead to publication of their work in the end. Others involve students in ongoing projects or use them as grunt workers to care for animals or have them clean their labs (try to avoid the latter two - you need research experience or involvement). All intermediates between these two extremes are possible.