Blisters are probably going to be an issue for many of you during long hikes on the trail. The reason they can be so problematic is they can manifest themselves when friction occurs. When your skin continuously rubs against either article of clothing or another surface, then blisters can occur.
This is why it can be beneficial to purchase some ointment or cream to help prevent blisters. Also, for the record, in more extreme cases, blood blisters can persist, and these are not very fun to deal with (especially when you still need to walk).