Normal definition television has only a certain amount of information in it and if you display it on a large screen you don't get any more detail it just gets bigger.
Imagine a picture from a newspaper - it's made of dots - If you blow it up huge you don't see any more, the dots just get larger and it starts to look blocky.
There are things called scalers. They take a low resolution picture from terrestrial TV and try to make it look like a high definition. They can't add any more information into the picture so the kind of make a best guess and break it up.
In the past they've been very expensive but have come down a lot recently - things like this:
http://www.tech.co.uk/home-entertainment/high- definition/news/ces-2007-three-1080p-upscalers -from-gefen?articleid=608051403