A sector is a group of companies that share product types. For example, Intel and AMD both specialise in computer processors and other computer-related technology, so they are part of the technology sector.
I chose to buy stocks in Nvidia and AMD. I chose Nvidia since they were releasing a new product and I expected their stock to go up. I invested in AMD after Nvidia had released their new product because their stock had gone way down so I could buy a lot for lower cost and sell it later for a profit.