I'm slowly reading through Bitcoin for developers course at saylor.org to refresh and perhaps learn some new things about Bitcoin. Here is an excerpt I liked...

"Digital Currencies Before Bitcoin

The emergence of viable digital money is closely linked to developments in cryptography. This is not surprising when one considers the fundamental challenges involved with using bits to represent value that can be exchanged for goods and services. Three basic questions for anyone accepting digital money are:

- Can I trust that the money is authentic and not counterfeit? - Can I trust that the digital money can only be spent once (known as the "double-spend" problem)? - Can I be sure that no one else can claim this money belongs to them and not me?"