In the beginning, when I first stumbled onto the EOS project I could just feel it was something big. Now grasping what that big thing was and how I could integrate myself into it was a rabbit hole I am still falling down. For those of you who have felt something similar I am writing this for you. For those of you who have those triple digit IQ's and can't figure out why the rest of us can't grasp "the obvious" go ahead and chuckle and feel free to sprinkle some bits and pieces of the big picture on us.
The first thing I believe that is important to understand is why the whole crowd funding is being done on the ethereum blockchain using ERC-20 tokens and how that is intended to play out. Mr Larimer and his team are thinking so far ahead of most of us that I think they fail to realize their "simple" explanations are only simple to them (and their kind). First, understand that there will be an EOS blockchain, but it does not yet exist. The "money" being raised is for the purpose of creating it. There will also be EOS tokens, but again they don't yet exist.
Everything about this project is thinking big, learning from all the experience that preceded it (bitcoin, ethereum, dash to name a few). I believe bitshares and steemit are more akin to what these devs would think of as some example apps that can and will run on the EOS platform (when it exists). Learning from ethereum's successes and failures, they have elected to create a few "apps" before the OS in an attempt to learn better what will and what will not function on the OS. . . before starting the engine. Think of it like R&D, they are about to create a major new concept in blockchain operating system and they are trying to anticipate what the app developers will then want to create to run on it. . .and if and how it will work? Now having learned a great deal from the bitshares and steemit projects 
As for the ICO, the impression I get is that they looked very closely at previous ICO crowd funding methods and decided to create a sort of hybrid method using the best parts of others while recreating what didn't work so well. Rather than slapping together a simple blockchain to sell EOS tokens on, they looked around and saw that they could use an ethereum smart contract and ERC-20 tokens to represent the place holder for the value of the soon to exist EOS tokens once the blockchain is up and running. What this does is give them the ability to allow others to invest in the project while simultaneously using the funds to develop it with. What you are in effect doing if you buy EOS ERC-20 tokens is entering into a smart contract agreement with the EOS team in which the contract guarantees you will receive EOS tokens on a specific date, at a specific time (when they will exist) This time is 48 hrs after the last 23 hour crowd funding period ends and all the 1,000,000,000 tokens (represented by ERC-20 tokens) have been distributed. The time triggers the smart contract to look into the ETH wallet you have "claimed" and store your EOS ERC-20 tokens in (remember these are just place holders representing the future EOS value), then it counts them, then instantly sends the same number of (newly created ) EOS tokens to the EOS wallet address you generated the keys to and mapped back to the contract.
Think of it like a three part puzzle: Part 1) your ETH wallet (existing on the ethereum blockchain) holding your ERC-20 tokens as "value" place holders (lets say metamask). Part 2) the smart contract itself, also on the ethereum blockchain, which contains the rules of your agreement. And part 3) the EOS wallet (keys you generated, though the address can't actually yet exist because the blockchain it will be on does not yet exist). The contract being "mapped" to both the wallets; existing and soon to exist, just means that it knows where to look to see how many place holders (ERC-20 tokens) you purchased, and also knows where to deposit the newly created EOS tokens (the address you created and mapped that does not yet exist, but will). This contract contains all the ETH addresses and all the future EOS addresses of all the participants who have contributed ETH to fund the project.
This will be the genesis block, which is brilliant really, because there are no pre-mined tokens like the wallet on bitcoin, for example, that contains a million BTC and purportedly belongs to Satoshi Nakamoto. They is no secret whale! Instead, the sum total of everyone who bought ERC-20 tokens becomes the genesis block. No one can accuse any of the devs of front running because it is all transparent on the blockchain. This whole project was so meticulously thought out and designed that it has essentially confused everyone without an IQ in the triple digits.
The basics of becoming a part of this project, which in my opinion you would be a fool not to, are really simple once you understand what I have just outlined above. You buy EOS ERC-20 tokens with ETH, "claim" them to the ETH wallet of your choice (it does have to be one that can communicate with the smart contract, thus myetherwallet and metamask being suggested, metamask being the easiest in my opinion), generate a set of EOS address keys (and backup and guard the private keys like they are valuable. . . because they are), then map the addresses to the contract so when the contract is activated it can count the ERC-20 tokens in the one, and deposit the EOS token into the other.
At this point the ERC-20 tokens have served their purpose and die. This is when the real fun begins, because now EOS is alive and breathing and you have EOS tokens on the genesis block. . .You want to miss being apart of this?
I will soon put together a step by step EOS ICO buying guide using metamask and the EOS app, There are a few really good guides for doing so with myetherwallet like the one by 
peace
jax