What's a difference between Simple Queue and Queue Tree?

Hi this is your personal google service:

https://wiki.mikrotik.com/wiki/Manual:Queue

There are two different ways how to configure queues in RouterOS:

/queue simple menu - designed to ease configuration of simple, everyday queuing tasks (such as single client upload/download limitation, p2p traffic limitation, etc.).
/queue tree menu - for implementing advanced queuing tasks (such as global prioritization policy, user group limitations). Requires marked packet flows from /ip firewall mangle facility.

It is somewhat complicated because they can both be used for the same sorts of things but they each have something they are particularly good for:

Simple queues - good for rate limiting for customers, networks, etc., so that they cannot pull more than their allocated amount
Queue trees - good for doing prioritization over bandwidth limited links (ex. fixed 50Mbps pipe) to ensure that high priority traffic makes it across the link if it is congested

You can use either for the function that the other normally does, but it just tends to be more work.

Queue trees and simple queues can both be used at the same time on the same device - the same packet will pass through both simple queue and queue tree hierarchies on a given router, and therefore you can use this for multiple levels of QoS.