File format plays very important role in almost every stream of science. Don't believe ? see this comprehensive list of different file format on Wikipedia : either subject wise or alphabetically. Indispensability of file format is only because like human beings any programme know very limited language and therefore can read only limited books :). In Bioinformatics, like any other subject, programs can read the data written in certain only therefore by the increasing the number of bioinformatics resources the number of file format or way to write a given data is also increasing day by day. Although it is impossible to cover all the file format in a single post I am trying to give the link for some bioinformatics resources and bioinformatics tutorials where different file formats are explained in detail. A very good list with detail description of most used file format can be found HERE.  Alternatively you can also see the different file format description in the PDF file given below written by TIGR.