I one sentence, these two security functions can be described as:
suPHP is a tool for executing PHP scripts with the permissions of their owners instead of the Apache user.
suEXEC provides Apache users the ability to run CGI and SSI (Server Side Includes) programs under user IDs different from the user ID of the calling web-server. Normally, when a CGI or SSI program executes, it runs as the same user who is running the web server.
Translated: suPHP is for PHP what is suEXEC for CGI.