Remember that time you shopped for a necklace online, and then jewelry ads followed you everywhere you went — from your Facebook page to news sites? You’re not imagining it: You’re being tracked.

The Internet is crawling with mechanisms for gathering info about users’ habits and interests. That provides valuable data for advertisers looking to target customers on an individual level.

How creepy tracking is depends on your point of view. For some, it’s an alarming invasion of privacy. Others say that same technology makes it easy to navigate the Web. And almost everyone acknowledges that ads pay for free content on the Internet.

The methods for collecting information about you are many and ever-evolving. Here’s a look at two very common tactics: cookies and user IDs.

Source: Nina Hale Inc., Electronic Frontier Foundation, Digital Advertising Alliance