Are fonts in ImageMagick system dependent, or do they come inside ImageMagick and can be equally used in any platform.
The fonts that are available on a particular system are system-dependent (for example, on Windows you're likely to have many .ttf (TrueType) and .otf (Open TrueType) fonts; on Mac, you're likely to also have PostScript fonts; and on Linux, you'll have X11 fonts).
But some types of fonts can be copied from system to system and will be supported - read on...
You can list all the fonts currently available to Imagemagick with -list font option (for versions prior to 6.3.6, use 'type' instead of 'font').
Different font types have varying levels of support in operating systems - even if the file itself is not OS-specific, the OS must have support for rendering the font. TTF and OTF are reasonably universally supported now; PostScript (as far as I know) require special tools for Windows and not all applications can use them; and X11 fonts' support is limited to various flavors of Unix.
Even if a font is supported, the way to render it can differ between operating systems and even between different libraries. When Safari 3 for Windows came out, the difference in antialiased rendering between Windows' ClearType and Apple's method was very visible.