Elias A. answered 04/19/19
Critically Acclaimed Music Producer & Artist | Logic Pro X
Hey there! First thing you'll want to understand is the difference between the two. If you're a musician and want to create songs or just hear your recordings, i would recommend getting a simple interface to start.
An audio interface is going to allow you to record audio directly into your computer's audio software (Logic Pro X, GarageBand, Ableton, Protools, etc.). This will be great if you play an instrument and want to be able to record your self with a microphone or directly with a cord (electric guitar, bass guitar, microphone, etc)
A mixer is going to allow you to plug multiple instruments or microphones in and adjust the volume, pan, EQ (treble, mid, bass) and in some cases compression, for each track, and play them through speakers that are connected to the mixer. In some cases, a mixer will also have interface capabilities, allowing you to record directly to your computer from the mixer but those tend to be more expensive.
I hope this helps! Feel free to contact me directly if you have any other questions about this or music production in general. I teach Logic Pro X lessons so if you decide to go with the interface, i'd be happy to show you how to use it and record yourself!