What ia allowed in the making up of a domain name

Hi
Which of the following symbols is allowed in making up a Domain name ?
Dashes -
Underscores _
Slashes /
Dots .
Also whch extensions are allowed in a Domain name ?
.com
.net
.ca etc.
Thanks

you can only use numbers, letters and dashes, but dashes can't be in the beginning or the end of the domain name