MS Word and WordPad fall into a specific category of software known as 'word processors'. These software are used to create high quality documents, official and informal letters, envelopes etc. Word processors allow editing and formatting of the data that you enter. MS Word is a fully featured word processor, while WordPad is a basic word processor with limited functionalities. NotePad is a 'text editor', which can be seen as a very basic version of a word processor. A text editor doesn't allow modification or formatting of data that you enter. 
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Word pad Is a Basic Format of Microsoft Word