Help? Separating Objects / Vertices

Blender Noob here -

How do you seperate vertices into separate objects and/or link objects 
into single objects?

Lets say, in edit mode, you created two meshes, but they're all part of 
one object, and now you want to seperate the meshes into separate and 
distinct objects.  How can I select 10 vertices and say, unlink these 
from this object and create a new object from them....

And then, how do you do the reverse?  Take these two objects and make 
them one so you can edit vertices from both while in edit mode for one..

Anyone?