If you're a programmer, I highly recommend trying to implement your own (toy) Bitcoin client -- just as a learning experience and not for general use or distribution. (Just be sure to advertise it as such if you put it up on Github). I've been involved with Bitcoin since early 2011 and am just now understanding the protocol in depth, having started to implement my own Bitcoin client.

You're best bet to do so is probably to just skip the reference client and its complex C++ implementation, and learn the protocol from the Python implementation [0], combined with the protocol specification on the Bitcoin wiki [1].

If Java is your thing, I've also heard good things about the readability of Bitcoinj, which is used for various Android Bitcoin clients. Just note that Bitcoinj implements SPV, which is not the full reference protocol. But it would still be a great start to learning about Bitcoin.

If you're not a programmer, there are a 101 different intro to Bitcoin sites. I think I've heard good things about the Khan Academy's intro videos, but I've never watched them myself. And yes, and Satoshi's original paper.

0. https://github.com/jgarzik/python-bitcoinlib

1. https://en.bitcoin.it/wiki/Protocol_specification

2. http://code.google.com/p/bitcoinj/

3. bitcoin.org/bitcoin.pdf

EDIT: s/legibility/readability

I learned most of what I know directly from the original paper: http://bitcoin.org/bitcoin.pdf

It's not too bad to read. Only the first six sections are really necessary to understand how it works.

Browsing Satoshi's mailing list and Bitcointalk posts is a good next step.

http://www.mail-archive.com/search?l=cryptography@metzdowd.c...

https://bitcointalk.org/index.php?action=profile;u=3;sa=show...

There are tons and tons of wiki and blog posts about bitcoin out there, but in my opinion the original paper is really good, if you have a technical background. I don't mean like crypto-expert, I think it's something the average programmer can read, maybe with a bit of googling. I usually wouldn't recommend a paper, but I found it really clear and straightforward. Maybe it's because I already understood bitcoin pretty well, but here it is if you want to take a look: http://bitcoin.org/bitcoin.pdf

The bitcoin wiki is also pretty good: https://en.bitcoin.it/wiki/Introduction

I wrote a comprehensive article on addresses and how they work. I've even included the BitcoinJS JavaScript library on the page so that you can experiment and follow along. Here it is: http://procbits.com/2013/08/27/generating-a-bitcoin-address-...

I see this kind of actitude from more and more people, seems to me the early adopters phase is about to end. Bitcoin is mainstream now.

I won't add more resources to the list others presented but i will +1 this suggestion: if programmer, code something bitcoin related.