You can share your Raspberry Pi's files and folders across a network using a piece of software called Samba, a Linux implementation of the Server Message Block protocol. You'll need to install this software:
Samba contains the SMB protocol, support for the Windows naming service (WINS), and support for joining Windows workgroups. A workgroup is a group of computers on a local network that can access eachother's folders. Samba-common-bin contains a tool that you'll need to register users with Samba. Once these packages have finished installing, you need to edit the Samba configuration file:
Find the entries for workgroup and wins support, and set them up as follows:
wins support = yes
The name of the workgroup can be anything you want, as long as it only contains alphabetical characters, and it matches the name of the workgroup that you want to join.
You also need to add the following section of code to smb.conf:
[pihome] comment= Pi Home path=/home/pi browseable=Yes writeable=Yes only guest=no create mask=0777 directory mask=0777 public=no
Scroll down smb.conf until you see a section called Share Definitions, and add this code there. The path should point to the drive or folder that you want to share. I've set 'only guest' and 'public' to 'no' so that Samba prompts for a password when I visit the folder that I've shared. This means that when I'm using a Windows PC, I can login to the shared folders on my Pi, and I'll have the same read/write permissions that user pi has.
Now type this command in a terminal, and enter pi's password twice:
If you have a PC or laptop connected to your workgroup, you should be able to see your Raspberry Pi in Windows Explorer under Network.