Pleadings is a pre-trial procedure which documents are filed and exchanged between the plaintiff and the defendant and which state the claims and the defences in the dispute.
Discovery of documents is a pre-trial procedure which requires the parties to list all the documents they have that are relevant to the case. Copies of the documents are normally provided to the other party.
Essentially, pleadings require specific court-required documents to be submitted and exchanged between the parties, with the most important being a statement of claim (document with claims by the plaintiff and remedy sought) and a defence (document which details the defendant's response to each of the plaintiff's claims.)
Discovery, however, does not involve specific documents, it involves documents which will simply become relevant to the case, such as a copy of the contract in dispute, or medical records if the plaintiff has suffered an injury as a result of the actions or omissions of the plaintiff.
I hope this helps!