Conspiracy is defined in Article 105 of the Penal Code and generally occurs where a person intends to committ a crime and agrees with one or more other persons to engage in the performance of such conduct. All conspiracies require an underlying criminal goal and an overt act by one or more of the conspirators. A conspiracy to sell drugs -- without regard to degree - means that the personal charged intended to engage in the criminal sale if a controlled substance and agreed with at least one person to do so and at least one of them did more than just talk about.
A person can be charged with conspiracy under a number of circumstances. Common example include where the police have the individual under arrest who actually made the sale and that person confesses that he had co-conspirators. Another common example is where the police have intelligence on a group that is preparing to commit a crime and arrest the group members for conspiracy prior to the crime occuring.