The waterfall model or the Liner Sequential Life Cycle model is followed in a sequential format. The process of Software development is divided into phases. In this model, each phase has to be completed before the beginning of the next phase. It is one of the earliest Software Development Life Cycle (SDLC) approach for software development. Since it is structured, it may be rigid. The process of software development is completed in a single project. Due to its rigidity, you cannot change the requirements after the project is in the development phase.
Agile, on the other hand, follows the incremental approach of software development. It helps in the continuous repetition of the development and the testing phase in the process of software development. With the help of this model, the lifecycle of the project development is separated into sprints. It is a combination of incremental as well as iterative process models that focuses on customer satisfaction and product adaptability by rapid delivery of the software product. It is extremely flexible. It can be considered as a collection of multiple projects. Since it is a flexible method, you can make changes in the requirements of the project development even after the initial planning phase is completed.
To learn in-depth about the Agile model, you must register for an online Agile Training.
Also, watch this video about the difference between Agile and Waterfall: