What is the difference between a "plaint" and "suit"?
Well, because your question is posted under "Law and Politics," I will assume that you are speaking about the terminology in those genres. I also am going to limit my answer to their usage in the United States of America in regards to the difference between a "plaint" and a "suit."
Please realize that the word "plaint" is not used much in American Law. Instead, we have a definition for "complaint" and a "plaintiff." The good news is, it pretty much means the same thing. A complaint is the very first paperwork or document that is put on file with the court (usually by the Clerk), and it is done by the person/business/organization that is claiming their right against another person/business/organization. The person who is filing the complaint, in United States Law, is called the plaintiff. The person/business/organization on the defensive (who didn't put in the complaint) is called the defendant.
The word "suit," again, used less often in the United States, is just a shortened form of the word lawsuit. Further, it's a really general term for ANY legal filing. The word "lawsuit" is a very common term, again, for any legal filing that involves a case to be figured out within a court of law.
A "suit" refers to a lawsuit. It is a civil action, as opposed to a criminal one. A suit is brought by one person or group (plaintiff) against another (defendant), seeking damages for action's of the defandant.
A "plaint" is commonly defined as a summons, a legal document sent to representatives in a lawsuit. It informs a defendant of the legal action being brought against them, and provides case number, court dates, etc..
In some countries, the term "plaint" is more than a summons, and is used in just the way that "suit" is used. See the defintion provided by the link below. Plaint is not a term used int the US, but mostly in English law.