Files with DWG extension represent proprietary binary files used for containing 2D and 3D design data. Like DXF, which are ASCII files, DWG represent the binary file format for CAD (Computer Aided Design) drawings. It contains vector image and metadata for representation of contents of CAD files.
Files with OTT extension represent template documents generated by applications in compliance with the OASIS' OpenDocument standard format. These are created with word processor applications such as free OpenOffice Writer and can hold settings that can be used to generate new documents from these template files. These settings include page margins, borders, headers, footers, and other page settings. Such templates are used in official documents such as company letterheads and standardized forms.
You can also convert DWG into many other file formats. Please see the complete list below.