"Set Startup" under System 6 lets you switch between the Finder and
MultiFinder and set certain applications and desk accessories to open
automatically when you turn on your Macintosh.
System 7 has added more flexibility to this feature. Using System 7, you
can have applications, documents, control panels, desk accessories, or
aliases open automatically when you start you Macintosh.
To do this, just put the items you want opened into the Startup Items
folder inside the System folder. If you want an application to open at
startup but you don't want to move it from its current location, just make
an alias of it and put that into the Startup Items folder.