App stores typically take the form of an online store, where users can browse through these different app categories, view information about each app (such as reviews or ratings), and acquire the app (including app purchase, if necessary – many apps are offered at no cost). The selected app is offered as an automatic download, after which the app installs. Some app stores may also include a system to automatically remove an installed program from devices under certain conditions, with the goal of protecting the user against malicious software. Many app stores are curated by their owners, requiring that submissions of prospective apps go through an approval process. These apps are inspected for compliance with certain guidelines (such as those for quality control and censorship), including the requirement that a commission be collected on each sale of a paid app. With the ease of use apps offer, and their presence on most mobile devices, app stores rose to prominence at the beginning of the 21st century with their adoption by iOS (iOS App Store) and Android (Google Play). Similar systems for the distribution of apps written for other operating systems have also been available for some time (particularly Linux distributions since the early 1990s), through package management systems and their graphical front-ends.