WhatsApp is a messaging app and service that allows users to exchange messages, images, audio, and video messages over the internet. It is a cross-platform app, allowing users to communicate with each other regardless of their device type. WhatsApp also offers a range of features such as group chats, voice and video calls, document sharing, and location sharing. WhatsApp is a secure and reliable messaging service that is free to download and use, making it a popular choice for staying in touch with friends and family.