Acquisition is the purchasing of all the company’s shares or most of it by another company. So, company-buyer can control the bought company. For startup companies, the acquisition is the way to be taken to the next level or to survive.
A lot of startup founders are satisfied to sell the founded company to a bigger company and get a reasonable profit, in case they don’t have a possibility to continue to develop the company because of lack of funding or experience, etc. A startup could be bought by a venture capitalist or a competitor or as an intellectual property (patent or trademark).
See also Buyout.