Got Picasa Photo Viewer -- by accident

So I was looking around at photo sites.  Wanting some place extra to store photos.  I somehow now have Picasa Photo Viewer on my computer with a copy of all my photos.

What is this?

I'm surprised there was enough room on my computer's hard drive to copy all my photos.  I thought I was almost out of room.  Maybe this Picasa works different?

HELP please:-O.