webconfig is a file that allows you to customize settings for the server
machine. you may also use it to customize settings for the application
running on the server machine. it is readonly for the most part. it comes in
very handy when you want to change your application behavior without doing
it thru code. it can be used to override machine specific settings, security
etc.
global asax file is the same as a form aspx file except that it's contents
get compiled into a dll where the variables service the entire application,
unlike form variables which service the particular form page - hence the
name global.
Touching any of these two files will cause the application to restart.
regards