Asthenosphere can act like a viscous liquid right?

Question

Is lithosphere part of the crust, or just part of the mantle?
The core is the most solid part within the earth right? P-waves can travel through all the layers, except for S-waves right?

Also What really is the plate tectonic theory. I want to know like what it means. For example the continental drift theory indicates that the continents have moved slowly during time. They were once joined together and formed a big land mass known as Pangea, and slowly drifted apart.
Does the plate tectonic theory revolve around diverging, converging and transforming plates? Id like to know the actual definition of what the plate tectonic theory is, thanks. Also Ive checked on the internet, but didn't find the answer I longed for, so could you please explain to me about this, thank you very much for the help:)

Answer 1

I have no credentials in geology but can suggest this as a good introduction to plate tectonics.

http://www.ucmp.berkeley.edu/geology/techist.html

The theory was first proposed by Alfred Wegener of Germany (who also was not a geologist). Some aspects of the theory have changed with time, but he was basically right.

The plates move along major fault lines and subduction zones, where one plate plunges under the other.