Just to start off,
Can anyone explain the difference between a dialect and a variety of language? Or are they one in the same?
I'll give you the definitions I'm familiar with at a tertiary level, though keep in mind that I didn't do Englang and there are some things that this subject modifies for the sake of clarity.
Broadly speaking, varieties of language are
mutually intelligible. For example, the variety of language that you use to address your friends in informal settings
is something that your grandparents could understand, even if they don't really speak that variety. Likewise, you could listen to
AAVE and you'd get the general gist of it.
Dialects on the other hand, are more
distinct varieties that (cross-linguistically)
may or may not be mutually intelligible. Unfortunately, English doesn't really have as clear a distinction between dialects and variants, which is why you will probably see the terms used interchangeably in textbooks or analyses. The best way to think about it is like this:
Let's say there are five different language clusters, represented as A-E. Now, each of these communities are fairly shy, and they don't like fraternising outside of their immediate area. As such:
• A tribe only converses with B tribe
• B tribe converses with A and C tribes
• C tribe converses with B and D tribes
• D tribe converses with C and E tribes
• E tribe only converses with D tribe
However, once in a blue moon, all the tribes meet up for an inter-valley market or something, and all the tribes get a chance to mingle. BUT
• A tribe can't understand C, D, or E tribes
• B tribe can't understand D or E tribes
• C tribe can't understand A or E tribes
• D tribe can't understand A or B tribes
• E tribe can't understand A, B, or C tribes
Suppose you're a linguist tasked with classifying this/these language(s) - what do you do? Are they all speaking different languages? If so, why can C tribe understand D tribe? Or are they dialects of the same language? In that case, why can't A tribe understand E tribe?
This is a problem English doesn't really have to face, as you could gather up a Australian, New Zealander, Scottish, British, and American speaker, and outside of the really outrageous accents, they'd probably be able to understand one another. But in other languages, the differences are such that if you take one speaker (from 'tribe A') and another (from 'tribe E') they'd have no idea what the other person was saying. For example, if you took someone from Sicily in the south of Italy and made them listen to a speaker from Vincenza in the north (and the Vincenzan speaker was talking as they would with their Vincenzan pals rather than in 'Standard Italian') then the Sicilian would have a fairly tough time working out what was going on.
Both 'dialect' and 'variety' are kind of umbrella terms used to capture some highly diverse kinds of language contrasts, though, so the use of those terms is sometimes a bit idiosyncratic... I've had two lecturers within the one subject take completely different stances on what constitutes a 'dialect' as opposed to a 'variety,' so there's not a lot of consensus here, in my experience
tldr: varieties are more socially determined language clusters that are used amongst speaking communities; dialects tend to be more historically/geographically/politically determined distinctions. Tbh my impression is that whilst one is often more appropriate (and you'll probably look at varieties more often than dialects, so I'm guessing it's the former), an Englang assessor wouldn't be confused if you were using the other