Twill, poplin, linen, whats the difference?

I'm trying to buy some crop pants from Victoria's Secret that I will be wearing while volunteering in rural areas of the Philippines. I just need a casual set of pants that are loose enough so I don't boil in the heat but they need to be about mid-thigh or longer. There are all these fabrics they are made of and I'm wondering what the difference is between twill, poplin, linen or seersucker? And which would be most appropriate for casual volunteering?