In the previous videos, you've learned what data-based advertising is and where the data use to optimize ads comes from. By the end of this video, you'll have a better understanding of what data actually is and what types of data records exist. Let's get started. As you most likely know, every website is made of code and that code is stored on the publisher's web server. Every time someone interacts with the website, they tell the server which pieces of the code to display. Every request they make for pieces of code or elements of the website leaves a record in the server. That record is referred to as the web server log and it looks like this. Basically, it looks like a string of code. This code may look foreign to us, but it's not too difficult to understand its components. First, you see a series of numbers, and that's the person's IP address. It tells the server where to send the data. Next is a unique identifier. This server recognizes who is asking for the information. This identifier is typically pulled from the person's browser, and it's usually a sequence of characters. Note that this is not personally identifiable information. No names or physical addresses are stored here. Next, if the website requires the person to log in, there may be a username here. Next, there's a data and a timestamp of when the information was requested from the server. After that, it's a string of code that identifies what information the person is requesting. This is how the server finds the right piece of data to return, the string of codes embedded in the links on the website so that you can click on a link and given instruction to the server at the same time. Next, you will see a number that tells us whether the information was successfully provided to the person. 200 means successfully delivered, while 404 means error. Finally, another number reflects the size of the content file, that the person has received. Every interaction leaves a trace in the publisher and advertiser's servers, the servers often also send some information back to the person and stored in that browser as a cookie. A cookie is a formative piece of text stored in a person's browser. Cookies contain information on when the person accessed the website, saw an ad, and when they clicked. Sometimes login information is stored as well. Cookies allow a server to connect multiple interactions from the same person. This diagram shows how they work. The person requests to page through the browser. To the server, it looks like a browser requesting a page. When the server gets this request, it sends the page to the person asked for back, but alongside the page, it also sends a tiny text file that is placed inside the person's browser. The next time the person requests a page from the same server, requests will come with a cookie attached. Now the server has a little bit of information about the browser that's asking for the page. While cookies can be a great way to connect the person's interaction with the server and pass the information back and forth, they do have some challenges. Ideally, the cookie would function as the memory of the interaction between a website and a person, and with that memory intact, navigation would be smoother for the person. The website would know what actions they took, or it might remember settings they like, or pages and ads they saw. But people don't use just one browser for all their interactions with the website. They may use different computers, different browsers and mobile devices. Cookies don't walk across devices and the memory trace is lost. People can also erase cookies from the browser, which erases the memory for the publisher and the advertiser. One of the major complications with cookies is that they don't work on mobile apps. As a result, it's hard for a publisher to link behavior from a person on its website, to behavior on its app. For websites where people log in, a user identifier, a UID can be created. A UID is a unique number assigned to someone's account that can be associated with all their logged in behavior. All interactions, regardless of browser or device, can be connected to UID. With a UID interests, behavior, and demographics can be stored. Anytime a person logs in, further behavior can be associated with them. It is important to note that UIDs use a number or text files, but they don't store personally identifiable information. It's also common practice in the digital world to hash the information. This means that the original data item gets translated, into a hash data item by applying an algorithm. As a result, what is stored is unrecognizable unless you have the hashing key, another algorithm that lets you revert the information back to its original form. This is an extra safety measure to help keep people's privacy intact. This video might have been a bit technical, but you now should have a good understanding for what types of data records are collected to customize user experience online. In the next video, we'll find out who owns, collects, and stores user data. See you there.