An ICO, or Initial Coin Offering is a mechanism for raising funds, under the terms of which future crypto currency is sold for liquid virtual money. You pay with FIAT currencies, bitcoin or ether in an ICO, and in return you receive tokens of the new project. This is an evolvement of the crowdfunding model.
The Pre-sale is the stage before the ICO (main stage) fundraising for the project.
A Pre-sale is conducted to test and verify the demand for the project, get support from the community, and expand the budget for marketing before the main ICO.
Not all ICO projects carry out pre-sales.