Auxes and Subgroups share a common concept—they both provide a way for one or more tracks (or busses) to send their signal to a single bus so that common signal processing can be applied to the mix of their signals.

Aux sends leave the existing signal routing to the main mix in place, and are typically used to create a separate mix to send to (for example) monitors or headphones (for performer monitor mixes):

Subgroups usually remove the original signal routing to the main mix and replace it with a new one that delivers the output of the subgroup bus to the main mix instead.