HD or High Definition. What is it?
And can you actually tell the difference between
an SD (standard definition) and a HD film?

Yes. There’s a huge difference. Technically speaking, High Definition
is when the resolution of a film is 1080p. Basically it means that
the resolution quality is significantly better, so that when you watch
a HD film, you will see crisp detail. (Not blurry fuzz). To compare HD
to SD, it's like watching the difference between a cinema movie and
a 1980’s home video. The same is true for Blu-ray. When we use
Blu-ray discs we are able to format movies at a higher quality than
when we use regular DVD discs.

Increasingly, people are switching to HD TV’s and Blu-ray systems.
If you haven’t yet, chances are you eventually will. And here is
where the long term value of having a HD Blu-ray film kicks in.
But be careful. Videographers may offer several 'versions' of HD,
and sometimes this is really just SD footage, reformatted. 

We only use professional Panasonic P2 HD film cameras at our
weddings. They're good. They're expensive. They're the real deal.