ICO are the concept of crypto currency world and IPO are of the stock market. The main difference between these 2 is that in an ICO anyone can take part and he can contribute any amount of money to the project, but in an IPO only so called accredited investors can take part and I don't admire it much as it causes manipulation by those taking part in the IPO. But these days the projects in cryptocurrency world are also becoming more like IPO, take Jibrel for an example they are having minimum cap per person set at $5k but that's too much for most of the people, I think they should decrease the minimum cap per person.