1. When you are making suggestions, they have the same meaning: What should we have for dinner? How about pizza?/What about pizza? 2. 'What about xx?' can also mean 'Wait a moment. Haven't you forgotten xx?' For example: All our cousins are coming to the party - Fred, Joe, Lisa and Ann. What about Harry? You haven't forgotten him, have you? No. I asked Harry, but he can't come. You can't use 'How about..?' in this sense. You can only use 'How about..?' for suggestions.

Used in a discussion in which one party wants to bring something new into the discussion that hasn't been thought of yet. "We have asked everyone and nobody knows how to do it" "What about Josue?" "Great idea - I'll call him now" Similarly if someone wants to raise a speciific case "Everyone on here is learning a language" "Really? What about Josue?" "Yeah, he's learning English" "What about" and "How about" are almost interchangeable in both of these contexts. In the UK we would definitely be more likely to use "What About" here. I think in the US they might go for "How about" but not every time. For example, there is an american film called "What About Bob?" http://www.imdb.com/title/tt0103241/ In American uses you might come across someone saying "Well how about that...." meaning "That's weird/surprising/impressive". This seems to be quite common in old black and white films like a 1950's equivalent of "Woah... Dude!"... "Mr Henderson, Mary Lou got hit by a raygun from the flying saucer we saw last night and now she has turned into a giant crab" "Well how about that!" ...But I think it is used today, even in situations where no giant crabs are involved. We don't really use this in the UK at all.