Here's the opening sentence of a paper about attacks on the Ethereum 2.0 consensus protocol:
The Proof-of-Stake (PoS) Ethereum consensus protocol is constructed by applying the finality gadget Casper FFG on top of the fork choice rule LMD GHOST, a flavor of the Greedy Heaviest-Observed Sub-Tree (GHOST) rule which considers only each participant's most recent vote (Latest Message Driven, LMD).
If that makes perfect sense to you then feel free to skip this chapter entirely. Otherwise, read on!
Our aim is to understand that sentence in all its parts. There's a lot to unpack, but we'll take time over it. We'll begin with some Preliminaries covering the basics of consensus. Then we will look in turn at each of the two consensus mechanisms used by Ethereum's proof of stake protocol, starting with Casper FFG, which is used to achieve finality, and then LMD GHOST which provides slot by slot liveness. After considering them individually we will look at how they work together as the combined consensus protocol that's become known as Gasper.