some webhost offer unlimited domains if you choose them as your host. is this a good thing? will it expire? if you want a different host will this be a problem?

You should be able to find out all this information by contacting the webhost or the webhost company.

That just means that you can buy domains from other sites and then share bandwidth and storage for all of them. This is normal for hosting sites. Its also a useful feature so if you want a second or third site you only have to pay an extra 5 dollar a year. Price changes depending on where you buy the domain though.

You should be able to move your hosting when the free offer expires. I've done it without any problems.