In the 60's thru 80's "dope" usually meant marijuana. See the famous quote from the Fabulous Furry Freak Brothers ("Dope will get you through times of no money better than money will get you through times of no dope!"). For a time, "dope" became a more generic term, often used for heroin (which used to be called "smack" or "junk"). "Weed" is now usually preferred for marijuana.
These days, dope is most often used to refer to heroin, though it can refer to any drug that's common in a community.
"Man, you know where I can get some dope?"
"Dope? When did you start shooting up?"
"Oh, alright, smartass, WEED! After 30 years, old habits die hard! Can you hook me up, or not?"