At the $100k level, you'd be considered an "angel investor" in the tech world, meaning you are investing a relatively small amount of cash into a very-early-stage company. You'd be given equity and expected to advise the company, make connections to potential clients and VCs, etc. To put this in perspective, by the end of 2009, Twitter had already raised over $100MM in venture capital, so your $100k would have been laughed at, to be honest.
VC rounds usually start at around $1MM. Like with angel investment, VC is not just a money/equity swap. Venture Capitalists sit on the boards of the companies they invest in, so they are expected/required to have decades of relevant business/entrepreneurship experience. Even more importantly, VC is about connections, as startup founders don't only look at deal terms when comparing VC deals. Since money is the fuel that will propel their company out of "startup mode", they want the highest octane fuel they can get, meaning a sharp VC who will give good advice, connect them to potential clients, and eventually help them through a liquidity event (acquisition, IPO). It's called "smart money."
The answer to "what could it have entitled me" would have been totally up to the terms of the investment deal you made with Twitter. These are some VERY complex arrangements, involving esoteric clauses like "liquidation preference" and "capped participation". Google those terms and if you're not falling asleep reading their definitions, you might make a good angel investor.
Lastly, it's easy to look at Twitter's IPO and say, "I should have invested 4 years ago." What you should really ask yourself is "Do I know what will be making headlines in the business papers 4 years from now, and do I have access to these people?"
If all the above doesn't deter you from angel investing, you still have that $100-200k, and you wouldn't mind never seeing it again, here's a great article on becoming an angel investor:
http://www.paulgraham.com/angelinvesting.htmlLMK if you have any questions about this world. It's cutthroat and very oligarchic.
[Source: worked at various early-stage startups]