Cookie banner
A cookie banner is the notice a website shows on a visitor's first page view to explain which cookies and trackers it uses and to collect the visitor's choice before non-essential ones run.The banner is the visible part of a consent management platform. In the EU and the UK it exists because Article 5(3) of the ePrivacy Directive requires consent before information is stored on or read from a device, unless the storage is strictly necessary for a service the user asked for. In California and the other US states with a privacy law, the equivalent is an opt-out notice with a "Do Not Sell or Share My Personal Information" link rather than a consent request.
A banner that produces valid consent in Europe has a few fixed parts: a short text naming the purposes, an Accept all and a Reject all button of equal prominence on the first layer, a way to choose by category, a link to the cookie policy, and no optional category ticked in advance. Behind it, the scripts of each category must stay blocked until the matching choice, and the choice must be stored and recorded.
What a banner is not: a disclaimer that says "by using this site you accept cookies". Regulators have treated that wording as no consent at all since the Court of Justice's Planet49 judgment (2019) and the EDPB's Guidelines 05/2020. Whether a given site needs one depends on what it loads; see do I need a cookie banner.