What is the difference between a blog and HubPages?

A blog is a whole website. A Hub is just a webpage.

A blog contains lots of "posts" - each post is a webpage. A post on a blog could look exactly like a Hub, if the blogger wanted it to, but usually blog posts are a bit shorter.

When blogs were first invented, they were meant for people to use like a journal or diary, which is why posts are dated with the most recent ones at the top. These days, you can do lots of other things with a blog - many people hide the dates, and arrange their posts in categories (instead of in date order) - just like HubPages.

The difference between HubPages and a blog is that here, your articles are part of a big site, even though they're on your account. Take a look at any blog on Blogger, for instance, and you won't see links to anyone else's blog, only to other posts on your OWN blog. Whereas on HubPages there are lots of links to other people's Hubs. That makes it much less likely that people will follow your writing from one Hub to the next.

Also when blogs started, you could write about your life or anything you felt like. You can still do that, but probably no one will read it. For a blog to be successful today, you MUST choose a topic to specialise in, and stick to it.