Servo is a project to build a new rendering engine in Rust, taking advantage of its security properties and ease of doing parallelism to do things that other rendering engines are not willing or are not able to do in terms of parallel execution.

Quantum is a project to significantly change some of the core parts of Gecko. Some of this will involve sharing some code with Servo, some will involve changing the Gecko code directly.

There is overlap between the two in the form of some shared components, which may be causing your confusion.