Thanx, that's what I thought. VSTO never made any sense.

To answer the original question

VSTO is technology that enables writing managed add-ins that work with
office, this comes bundled with design time support for office with
Visual Studio, Winforms, WPF support. You can go ahead and easily
write both Application level and Document Level customizations or
addins using VSTO.
VSTA is a more generic technology that enables ISVs (Independent
Vendors) to harness the power of visual studio ide and .Net framework
Add-Ins framework to create a customization story for their hosts.
Lets assume i create an application like XML editor and i need to
enable independent developers, other companies to wrote add-ins
against the object model exposed by my XML editor, I could easily
integrate this flexibility into my host using VSTA, this would enable
the developers to use Visual Studio based Editor (currently similiar
to VS express SKU in VSTA 2.0) to easily create add-ins for my Xml
Editor (any host in a more generic sense).
VSTO is built over VSTA technology , you could say VSTO is VSTA but
tailored for office as an host application

to know more about VSTA
blogs.msdn.com/vsta