With the Everquest FtP...

You're limited on the classes you can make, and the races you can choose.
But you have access to every character already on the account, previous to a certain date.
My accounts of six or seven toons each, and the FtP limit is four, so I won't be able to make new characters with those accounts... unless I buy a slot, go gold for a month, or delete something to make space.

You get access to all expansions, except the current and previous.
So you can use any feature that was introduced in those expansions you have.
Upgrading expansions is separate from a paid 'Gold Level' subscription; if you buy an expansion, you are 'Gold' for a month.
If you purchase 'Silver' or an expansion, or were ever 'Gold' you are considered 'Silver' from there on in.

'Bronze' accounts are rather limited.
'Silver' accounts less so.
'Gold' accounts don't have those limitations.

AAs are a big part of Everquest.
It is assumed you'll have so many, by a given level bracket.
If you farm more than that, you'll be strong for your level.
If you don't farm enough, then you'll be weak for your level.
A 'Bronze' account can have 250 AAs.
A 'Silver' account can have 1,000 AAs.
Gold doesn't have a limit, and some accounts have 8,000+ AAs.
Somewhere around 70th or 80th (FtP cap is 90th), you reach a point where your class should have over 1000 AAs to remain competitive.
If you're going to play enough... going 'Gold' for a month, removes the AA cap and you can keep and use anything you've earnt.
If you're not going to play all that much, you can purchase a higher AA cap, I believe +100 AAs.

Everything on the SOE store is priced in Sony's cash.
Which you can buy at basically $1 = $100 SOE cash.
You can buy:
- extra character slot 800 ($8.00)
- a class for 750 ($7.50)
- a race for 300 ($3.00)
- BoA/Heirloom gear bundle for 500 ($5.00)
- Extra bag slot for 250 ($2.50) > I have access to six of my bag slots, meaning I cannot access my (as a Silver FtP) J.Boots or Mana Stone (which were in slots 7 & 8).
- Character Name change for 1500 ($15.00)
- Pet rename for 1000 ($10.00) > which makes it much easier for a healer to target your pet, if the pet will be tanking...
- Rank 2 spells, for 750 ($7.50) > You need R3 for raids; R1 are doable for all other content.
- +5 Task (Quest Log) cap, for 100 ($1.00)

Krono is something they have recently introduced.
It is bought with real money, and used in game to add a month of 'Gold' level subscription (and consumed in the process).
So if you make enough platinum through play, you can have a 'Gold' level subscription without spending any cash.
Alternatively, you can legally (within the EQ universe) purchase an item that will get you in-game currency.