The Bureau of Narcotics is a law enforcement agency in Grand Theft Auto IV based on the DEA. Like the DEA, their main purpose is to combat drug smuggling and use. They feature no prominent role in GTA IV, and are only mentioned in a news report on The Liberty Tree's website about a joint raid on a mobile meth lab (which takes the shape of a Boxville) with the LCPD and NOOSE. It is then revealed in another report that the Bureau seized the meth lab to educate people about the dangers of methamphetamine.
The only known agent is Albert Thomas, who acts as a spokesperson for the Bureau of Narcotics. His statements to The Liberty Tree imply that the Bureau is involved in illegal practices, such as helping cocaine smugglers in Vice City and producing and smoking methamphetamine.
It's possible that the Bureau is affiliated with the Drug Observation Agency, due to both of their involvement's in the combating of drugs. The only difference between the two is that the DOA is featured in GTA V as well.