How do mSATA drives differ from eSATA drives for my use?

The difference between the two is that mSATA drives are small internal drives for data storage while eSATA drives are external and are used to connect several external hard drives to a common system.

Can mSATA drives be used to hold operating systems?

Yes, if the storage capacity is large enough. They are typically 128 gigabytes or more.

Are all mSATA drives wired for Wi-Fi cards?

No. If you want to use your mSATA drive for Wi-Fi cards as well as internal storage, you'll need to choose one that is specifically wired for Wi-Fi cards.