Roman/Serif:
consistent line width, use of serif, “flat” angles (either 0 or 90 degrees) whenever possible.
Historical/Celtic:
More use of curves in serifs, triangular and somewhat extended serifs
More curves within the letters, as well as italicized and cursive.
Sans-serif/block
Lack of any serif, consistent and relatively thick line width
Blackletter/Old English
“doubling up” of lines, more curves, inconsistent thickness that can flow from thick to thin.