A SED, or self-encrypting drive, is a type of hard drive that automatically and continuously encrypts the data in it without any user interaction. What may surprise many is that a decent potion of the drives currently in the market are in fact SEDs. The method involves a Data Encryption Key that encrypts and decrypts data whenever data is written to the drive or read from it. But the process is so transparent to user that it is unlikely they would ever realize they have such a feature.
On an absolute basis, one device is clearly better than the other; but the expectations for what a Chromebook is supposed to do is so much lower that, relatively, Acer's C720 Chromebook feels like a better device than it really is. Asus' popular T100 budget hybrid, on the other hand, gets compared to other Windows laptops (or the iPad Air) and doesn't look as good in the comparison.