There are a few differences between true type font (ttf) and open type font (otf). Open type fonts are the latest generation of fonts. They support different platforms and more characters than true type does. Open type also allows designers more flexibility than true type does.
The difference between true type font and opentype font is that true type font only requires one file to be submitted and opentype font can be easily shared between operating systems. True type font will need a seprate file for every other font icon, also.