It was a volatile week in the stock market, with the S&P 500 rising 0.5% by Wednesday but finishing the week down a total of 0.5%. Tech stocks were hit even harder, ending the week down a total of 0.7%.

Several tech stocks made interesting headlines this week. Two of the stories prompted sharp sell-offs in shares of Netflix (NASDAQ:NFLX) and Roku (NASDAQ:ROKU), and the other was simple: Apple (NASDAQ:AAPL) started selling its latest iPhones.

Here's what investors should know about these stories.

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Image source: Getty Images.

Netflix stock falls

Shares of streaming-TV company Netflix fell hard on Friday after Netflix CEO Reed Hastings said that competition for the company was going to get much tougher in November. The CEO was referring to Apple's and Walt Disney's plans to launch new streaming services during the month.

"Direct-to-consumer [customers] will have a lot of choice," Hastings said in an interview with Variety.

A small sequential decline in paid domestic Netflix subscribers in the company's second quarter sparked some concerns among investors that competition was heating up. But management insisted in its second-quarter shareholder letter that competition wasn't likely a factor, because "there wasn't a material change in the competitive landscape during Q2."

Will a material change to the competitive landscape negatively impact Netflix's fourth quarter? Time will tell.

Roku stock plummets

Shares of Roku were slammed on Friday after one analyst initiated coverage on the streaming-TV platform specialist's stock with a sell rating and a $60 12-month price target. By the time the market closed, the stock was down 19%.

"We see dramatically more competition emerging that will likely drive the cost of [over-the-top] devices to zero and put material pressure on advertising revenue," said Pivotal analyst Jeffrey Wlodarczak.

The bearish outlook for the stock comes a few days after Comcast said its streaming device, Xfinity Flex, would be free for the company's internet-only customers.

Comcast's announcement on Wednesday and Pivotal's bearish note together sparked a total sell-off of Roku shares of 27% this week.

Apple's new iPhones

On Friday, Apple started selling the new iPhones that it announced earlier this month. This means the new phones have a week and a half to contribute to Apple's fiscal fourth-quarter results. Of course, Apple similarly started selling new iPhones on Sept. 21 last year. So the tech company will be up against a week and a half of new iPhone sales in the year-ago quarter as well.

Investors might not know much about how the new iPhones are selling until the company reports its fiscal fourth-quarter results. Apple CEO Tim Cook often gives some commentary on iPhone sales trends during the company's quarterly earnings calls. In addition, the revenue guidance included in Apple's fiscal fourth-quarter update should provide a clue as to how iPhone sales are doing, as the smartphone segment represents more than half of Apple's total revenue.

The tech company typically reports its fiscal fourth-quarter results around the end of October or the beginning of November.

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