Terms of Service agreements are documents that outlines rules that a user must follow in order to use a product or service. Whether or not a Terms of Service is legally binding depends on the document's purpose, construction, and legitimacy.
Terms of Service also deal with the limited liability of the company responsible for the product or service, and often contains details on the arbitration process for the resolution of legal disputes.
The sometimes unclear legal ground on which Terms of Services operate have attracted them a fair bit of controversy. ToS tend to be very long and difficult to read and understand for the layperson, and rarely give any meaningful options for an opt-out process. There have also been numerous issues with companies imposing unilateral amendment of their Terms of Service, where they change the content of the ToS without informing the other party; this has been found to be legally unenforceable in several regions around the world.
There have also been issues with websites like Instagram amending their Terms of Service to include language that implied personal data would be sold to other companies. While Instagram did eventually retract this clause, the practice has continued, and hiding privacy violations in lengthy legalese has become increasingly standard in the social media realm.