I am wondering, is there any way to easily share a folder between Linux and Windows?
If you want to share files from Windows, your file manager on Linux can probably connect to a Windows share with no extra effort on your part. Try browsing
smb:///. If you want access from the command line, run
mkdir /media/somedir sudo mount -t cifs //servername/sharename /media/somedir
(If you need help on the Windows side, ask on a Windows site.)
The answer is going to depend very much on which of two questions you are actually asking.
If you want to share files and directories over a network between a PC running windows and a computer running linux that are on at the same time, Gilles' answer regarding Samba is definitely the way to go.
If you're asking how to create a partition/drive on a single dual-booting system that both your windows and linux setups can read, the easiest way is to format a partition as NTFS and use ntfs-3g to mount it in read/write mode in linux.
For the Linux -> Windows part, the following is my favorite:
python2 -m SimpleHTTPServer
python3 -m http.server
This opens a HTTP-Server on Port 8000 for the current directory. You can access it from Windows by typing your IP-Adress in a browser - eg.
If you want to share a directory, you have to zip it.