Open Type is a scalable, cross-platform file format that is used for computer based typefaces. Adobe and Microsoft worked together to create a file format that has many benefits.
The two main benefits are:
Open type fonts have one of two file extensions (.otf, .ttf). Both work equally fine and you may search for Open Type fonts on many sites.
Key Features of an Open Type Font:Adobe Caslon Pro 18 pt Uppercase A. (Artwork below was captured from InDesign Glyph panel for display purposes only.)