Fragments are a core part of activities - not that much different. The use of fragments comes since Honeycomb 3.0 and the idea is an option to split the screen in several fragments at once. For example if you look at the gmail app for a tablet - you have one fragment on the left dealing with navigation and then the next fragment on the right is the list of emails.
On a mobile device, viewing area is limited, so you it could be said that fragments sort of behave like an activity - you interact with one fragment, which triggers another and so on and so fort. But in the end you always reference a super activity of each of these fragments - for example when you want to access the context.
So if you just want to wrap web pages in WebViews, stick with activities. If your scenario might involve developing for both tablets and phones, then go for the fragments.
Alternatively, you can read about the design philosophies of both here:
http://developer.android.com/guide/components/fragments.html
Good luck!