Sequencial Access: Occurs when data files are accessed in a sequencial/Linear manner, hence the name "Sequencial Access". Access time to data files is much slower than Direct access & is impractical when immediate access is required. Sentinel Values are used to indicate breaks in the data such as EOF(End Of File).
Eg: Magnetic Tape/Audio cassette
Direct/Random Access: Occurs when data files are accessed without having to read any of the preceding data to a particular data file, files can be accessed in any order. Data is accessed directly using an index. Access time to data files is much faster. Eg: HDD.