IFA stands for Identifiers for Advertisers, and are particularly relevant for the in-app world. These identifiers are maintained by the platforms they are on (usually Apple iOS or Google Android) and are useful for identifying a unique device across all apps on that device. It is typically inaccessible on the mobile browser though.
Like cookies, they are deterministic and consist of a string of 32 alphanumeric characters and are pseudonymous. Unlike cookies, they are controlled completely by the platforms they are on, and typically (with the exception of fraudulent device reset farms) have a long lifespan.
