The essence of conspiracy is an agreement . It doesn't have to be a written one. Usually, it's inferred from the facts or circumstances. Under some statutes, the conspiracy can involve any act injurious to public health, public morals, free commerce, or any act perverting justice. This probably won't be reduced by plea bargaining.
Conspiracy is something prosecutors love to prosecute.
There's a lot of presumptions and procedural rules that favor the prosecution. It's easy to get a conviction for conspiracy because, basically, all the prosecutor has to do is present all the evidence and let the judge tell the jury what test will be used to determine whether an agreement existed. In jurisdictions requiring an overt act, the standard is not as high as the law of attempt, and is basically proven by showing at least one of the conspirators had at least the intent to commit a substantive offense. Purpose can be inferred from circumstances surrounding the combination, such as failure to keep records, clandestine meetings, quantities involved, continuity of the relationship, etc. Proof of an unwritten understanding will suffice.