Antagonist: The character who tries to hinder the protagonist from achieving his or her aim.
Plot: Sequence of events, in other words, the things that happen in a story. In a classical story, the plot is usually instigated or provoked by the protagonist.
Sub-Plot: A sequence of events that happen parallel to the main plot. In a classical structure, the sub plot should either aid or hinder the Protagonist in achieving his/her goal.
Plot twist: A point in a story when things suddenly move in an unexpected direction, adding to the tension and suspense.
Sequence: A series of events in a plot which hang together and are connected, with a beginning, middle and an end or conclusion. For example, lets say you are writing a police detective story, about a detective hunting a criminal. You could have a Sequence where the detective discovers that a guy named Nino who lives in China Town might know where he can find his suspect. The detective then tries to find Nino and extract the information from him. The sequence would start off with someone telling him that he needs to talk to Nino. The detective seeks Nino. He finds him but Nino runs away. The detective chases and catches him. Nino refuses to talk, maybe he hates cops, he is loyal or scared, you know the drill. The detective threatens and maybe beats Nino up (the American way). Finally Nino talks, and tells him all he knows. The detective is now closer to finding his suspect. He says bye to Nino, gives him some money or knocks him out. This is the end of the sequence. The story moves on to the next sequence.
Acts: Distinct parts of a plot. Usually, a classical story is divided into three acts, which serve distinct dramatical purposes. Act 1, also called The Beginning, is about the first quarter of a story, Act 2 , The Middle, is about two quarters of the story in length and is in the middle of the story obviously. Act 3, the end is the last quarter of the story.
Don't worry too much about these terms yet, we will deal with them later.