Adware and Madware are advertising supported software that displays, typically unwanted advertisements, on your computer and mobile device, respectively. Adware uses your web browser to collect your history in order to target advertisements that seem tailored to your interests. Adware infections can span from a nuisance and annoyance, spamming you with pop-up ads, to outright malicious intent, with a malware download the goal of the intruding advertisement (you have a virus, click here to fix it).
The most common reason for adware is to collect information about you for the purpose of making advertising dollars. No matter what the adware is, it’s likely going to slow down your machine and/or even make it more prone to crashing while sucking time away from your goals and objectives of using that device.
Source: Norton
Additional Reading: Android Adware Tied to Undeletable Malware