As well said by the other contributors (rizwanr74 and Andrew Price) overall QoS is not something that can be covered into a forum post.
However to reply to your question MQC allow complex QoS policies (like nested policies) when compared with Legacy QoS (like rate-limit command, which you apply on a interface basis).
MQC (Modular QoS CLI) is by far the most adopted CLI by the vast majority of users, as it offers advanced capabilities such as configuration of traffic policing at three levels of policy map hierarchies.
The legacy CLI offers only a reduced set of the functionalities that MQC offers, and Cisco has always maintained both CLIs at parity to allow early users to seamlessly upgrade their Cisco IOS Software.
Because of the low usage of the legacy CLI, and given that MQC is recognized as the most complete CLI for configuration of QoS on its equipment, Cisco has reached the decision to deprecate the legacy CLI for QoS.
I found that somebody else asked the same question, but in a more specific way:
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