In addition to the extra bold capabilities you get in math with the
amsfonts option, you also gain access to the Fraktur and Blackboard
Bold typefaces. You select these with normal font-switching commands:
${\frak G}$ gives a Fraktur ``G'' and ${\Bbb Z}$ gives a
Blackboard Bold ``Z''. Fraktur will become bold in a bbox; there is no bold
version of Blackboard Bold. If you have the AMSFonts installed and have the
amsfonts option selected, example output can be found in Appendix A.