There are tons of discussions related to this topic on BTT, like this one regarding scam ICOs: https://bitcointalk.org/index.php?topic=1963768.0
ICO is a form of raising funds, that is very similar to crowdfunding (if you haven't heard of that one - it's when people are offered to invest in a certain project and receive stakes as a form of reward). ICOs are related to cryptocurrency world, the products of ICOs differ though, from medicine related projects to entertainments and AI. ICOs started launching in 2016, reaching top point in 2018: total number of ICOs 712, approximate amount raised by ICOs is 17.8 billion USD according to coinschedule data - https://www.coinschedule.com/stats.html?year=2018
Top 5 ICOs ever are Filecoin, Tezos, EOS, SIRIN LABS Token and Bancor: https://www.cnbc.com/2018/07/13/initial-coin-offering-ico-what-are-they-how-do-they-work.html