PC are nice in that they allow copying and pasting between different types of programs. Basically what this means is that when you copy some stuff in word all of the formatting is copied with the text. When you paste this in another program that program may or may not recognize what it is. In the case of pasting into the message box on this forum, it most likely has no idea what it is. But the person who wrote the copy and paste code was smart enough to allow the text to be removed from the formatting and be pasted.
Computers have no idea what text, pictures, excel data, etc. is. To a PC all that stuff is just 1's and 0's in an area of memory. It's up to the program to convert those 1's and 0's to something we find useful. Copy and paste is actually a very complicated program. In simple terms copying from word to this forum goes something like this:
User: "Hey copy and Paste here's a bunch of stuff for you to copy"
User: "Copy and paste put this stuff in the message box of the forum"
Copy and paste: "Hey forum here's a bunch of stuff for you"
Forum: "That's all garbage I don't know what to do with that"
Copy and paste: (looks at what it has)"Forum, there's a bunch of text in here can you use that"
Forum: "Sure I can take that"
Copy and paste: "Here's just the text, I won't give you the stuff that makes it garbage"
Forum: "Hey User, there was a bunch of garbage in that copy, I only took the text, here it is"
User: "Hey! that garbage was my formatting, @#$@% computer"