Auxes and Subgroups do share a common concept - they both provide a way for 1 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.
An aux send leaves the existing signal routing to the main mix inplace, and is typically used to create a separate mix to send to (for example) monitors or headphones (for performer monitor mixes).
Subgroups remove the original signal routing to the main mix, but create a new one that delivers the output of the subgroup bus to the main mix instead.
The following two diagrams compares the signal flow when using subgroups and when using aux sends:
