intrathecal question

I'm a childbirth educator and a new ob nurse. I work at a small hospital and we do intrathecals but not epidurals. I know the difference as far as effect on patients, but thats it. Where is the medication injected into for an intrathecal? Is there an actual intrathecal space or is it the subarachnoid space. From what I've read an epidural and intrathecal go into the same spot but the medications are different. Is this true? Any help would be appreciated so I can accurately answer my patients questions.
Thanks,
Cindy