Like it says on the tin.

I am starting this game with a new group who has played plenty of other stuff. I personally have only played through about 2/3rds of the way through the first scenario just to get a grasp of the rules, so I am not confident enough with the nuances to describe the classes so they can make an infromed decision as to what we want to play as.

Guardian- monster killer

Seeker- investigator

Rogue-resource heavy, sneak attacks? Card advantage?

Mystic- easy to say spell caster, but without a firm grasp of what spells do, not sure what that means.

Survivor-no idea. Tank? Also see some card advantage stuff?

If someone could give me a one sentence description of each that would be super helpful.

I think the cool thing with Arkham is that each individual character adds separate archetypes to their class making your choice in each investigator impact the rest of the team.

But streamlined the 5 Classes are pretty much what you described; Guardians are good at killing stuff and tanking damage with healing effects / Allies. Seekers are strong at gathering clues and provided other forms of support (healing/drawing cards). Rogues I feel are Jack of all trades they can be good at evading or fighting given the right builds. I would say they're common denominator among the two rogues is action efficiency, economy and maybe even combo reliant. Mystics are spell casters. And you can argue encounter Deck Manipulators. Survivors main theme would be win by losing. They have cards and abilities that help mitigate the chaos bag better than the others.

to me:

Guardians: Monster slaying and teamwork.
Seekers: Clue finding, solving the mystery.
Rogues: Highly adaptable, customizable. Often can get extra actions.
Mystic: Let's see if we can make Willpower do literally everything.
Survivors: Making the most of a bad situation, and stealth/evasion.

First I start off by saying there are two things to be done in the game: Kill/Neutralize Enemies, Get Clues. There is also a bit of surviving longer, which can be deck manipulation or damage mitigation/healing but all of that just lets you do the first two better.

Guardian - Kills enemies, protects other investigators. (mention protection as it gets them into the mindset of what they need to be doing in the game)

Seeker - Gathers clues, this is important because clues are what advances the story in positive way. (explains why clues are important)

Rogue - Has lots of resources, decent at killing enemies, good at avoiding enemies. Tends to struggle to get clues.

Mystic - Has spells, which makes them a bit of a swiss army knife until the specific spell runs out. They become as good as the other classes in their specialties but for less time. They also tend to have high risk/high reward cards and the only class that can manipulate the event deck. (I find players that don't like high risk cards do not play mystic well or enjoy it)

Survivor - Good at mitigating failure or turning failure into partial success. Pretty hardy/resilient/evasive as well. A fair amount of card draw.

Some additions to each role:

Guardian Deck: This deck has options to heal other players and prevent damage. However many cards are very expensive on ressources, so you must be careful not to include too many of these or you'll never play them. Investigators in this role have low sanity, but high health. Very few options to gather clues or boost intellect, so needs other players to gather clues.

Seeker has a lot of card draw effects, so depending on your deck, you'll hardly run out of options. This deck has no weapons and very few options to deal with monsters. Depends on others to handle these. High sanity, but low health.

The rogue deck is focused on speed and has many options to avoid enemies. However that's a bad idea, as it costs actions to avoid enemies every turn, so it's better to kill them. It also has a few weapons, but guardian weapons are better. It also has many options to gain ressources, but not many cards to make good use them them. It's a good support deck for characters, that have expensive cards and access to the rogue deck (like skids o'toole). Some cards grant extra actions, these are very powerful. Balanced sanity/health.

Mystic and survivor are generalists, that don't have a focus

Mystic cards fight and investigate mostly with willpower, however these cost ressources and have limited charges. In 3-4 player groups you run out of charges pretty quickly, so you either need aditional options for these from other decks or don't play mystic in big groups. In my opinion, it's the most difficult class to play. Characters have high sanity, but low health.

The Survivors deck has cards for combat and investigate. However they're also of limited use, so they're not very effective in 3-4 player games. Balanced sanity/health.

All characters have access to more than one deck, so you can mitigate their disadvantages to some point, by modifying their decks accordingly. However this requires some time to look up all approriate cards and some experience with deck building. I cannot recommend the base decks for big campaigns.

Here's My quick take:

Guardian: killing, protecting, healing health.

Seeker: clue gathering, card draw, & support for other players.

Survivor: less is more. But they all play very differently.

Mystic: killing, scrying, manipulation.

Rogue: evading or killing. Extra resources & actions.

Thanks everyone. This helped.