Learn English – “Hardly” vs. “barely”

"Rarely" and "seldom" are synonymous and interchangeable, used to indicate low frequency.

"Hardly" is not used to indicate infrequency (unless you say "hardly ever," which is synonymous with the other two) but rather the extent of a quality, as it is synonymous with "barely." For example:

The light was hardly visible.

...meaning you could only just see the light, as compared to:

The light was seldom visible.

...meaning you could only see the light sometimes, irrespective of how bright it was.

It depends on the context. If you are referring to it scientifically, you call it cannabis (usually cannabis indica or cannabis sativa). Marijuana is a generic term mostly used by non-users. People who smoke the drug have a seemingly inexhaustible supply of slang terms for their favorite recreational drug, of which weed and bud are currently in vogue. You would not be likely to use those terms unless you don't mind people assuming you use the drug yourself, because they very likely will make that assumption.

EDIT: You can use the term pot in a disinterested way, as someone pointed out. Weed flirts with crossing the line, however, and bud is tantamount to a term of endearment.