Cookies have been the bread and butter of the Internet advertising world for many years now. With the help of cookies companies are able to follow users' activity through the web, make inferences about their likes and dislikes, and then serve them highly targeted and relevant ads. Cookies are essentially used for convenience and to enhance the user experience online.
What exactly are cookies?
Cookies are small text files that are dropped on a user's web browser by a website that the user was viewing. Each cookie contains a small amount of information such as a unique identifier that links a user to a specific website. This allows for a more personalized experience for the user as the browser remembers the user. The information stored in a cookie and it's ability to track user activity on the web has been used by marketers and advertisers to serve more relevant advertisements to users and improve ad effectiveness.

So do cookies exist on mobile?
"Cookies don't exist on mobile" has been a long-standing misconception in the mobile advertising ecosystem. The existence and application of cookies in mobile has been heavily debated and is a source of a lot of confusion.
In a nutshell, yes cookies DO exist in mobile. The reach of cookies in mobile however, is limited.
Unlike the web, cookies are not as effective in mobile because they cannot be applied everywhere. Cookies in mobile are, at best, unreliable. With the mobile market increasing at record speed, overcoming the inefficiency of cookies in mobile has become one of the biggest challenges in the mobile ecosystem.
On the web, mobile cookies are placed on the browser and that is where all the ads are served. On mobile however, the user behavior is fragmented. Users access the web using the mobile browser, but also use an array of apps which have the ability to display ads.
It's because of apps, that we get cookie confusion. Lets look at each environment separately and understand the extent of cookies in each:

a) The Mobile Web:
Cookies do exist on the mobile web just as they do on the desktop. Users who browse the Internet using mobile web browsers get cookies placed on their browsers. Every mobile browser, just like desktop browsers, has different cookie settings and handle first party and third party cookies differently. In essence, cookies are fully functional on the mobile web. The main limitation of cookies on mobile browsers is that they reset when the browser is closed or when the phone is shut down/restarted.
b) The Mobile Apps:
Cookies also exist within apps when a browser is needed to view certain content or display an ad within an app. However, the cookies are completely "sandboxed" in apps. This means that cookies from one app cannot be shared with another app and that they remain private to each app. This is a handicap for mobile marketers as it is extremely difficult to track user activity & behavior across apps. Being able to track user activity and behavior is the foundation of ad targeting and thus the inability to do so makes it extremely challenging for mobile marketers to improve ad effectiveness.
With mobile app usage accounting for almost 86% of mobile users time as opposed to only 14% on the mobile web, the mobile advertising industry needs a replacement for the cookie that can be used to track user activity across apps. The mobile advertising ecosystem is still in its nascent stages and companies are still experimenting with different techniques to overcome the issue of inefficient and unreliable cookies in mobile. Even though some companies have had varying levels of success surpassing the cookie issue to track users across apps and different mobile devices, there is still not dependable way of doing so. It will take some time, but a way to replace cookies on mobile devices will emerge.