There is nothing describing the specific ranking system that the Resistance uses, but I would assume it’s more or less the same as that of the Old Republic or the Rebel Alliance. So:
A General is a high-ranking leader of an army or a starfighter command, with Admiral being the equivalent rank within the navy. Both are graded C-1. This is sometimes flexible, as there are multiple different kinds of Admirals, and it’s not always clear whether they are exactly equal in every scenario. Some Generals preferred to keep their title even when appointed to a fleet command.
Vice Admirals may or may not be equal to Generals, depending on whether the grade of C-1 can be applied to all Admiral positions. If not, a Vice Admiral would be inferior to a general. It is not clear which would be used in the Resistance, but if the latter is true then I would assume Holdo took command because of her affiliation with Leia, as sort of an informal temporary arrangement.
Admiral seems to be the highest rank anyone has so far held in the Resistance, but if an Admiral were to be promoted I would assume that they would take the rank of Admiral of the Fleet or Fleet Admiral, in accordance with the ranking system of the Rebellion.
The ranking order below these is often unclear and can vary wildly from faction to faction, so there’s no telling for sure how it goes in the Resistance, especially because we only have so many characters to work from. I hope that helps at least!
~ Jacen