Ok, this is where things get confusing. Igraine, Morgan, and Morgause are frequently conflated, expanded on, shaken, stirred, and served a la carte. I could go on and on referring to various "standards" of work, but even those aren't always terribly enlightening (Mallory, usually makes things confusing, for example, in his attempt to be comprehensive).
So, for simplicity, I'll summarize what The New Arthurian Encyclopedia says, which is hardly perfect, but is frequently used as an essential reference by Arthurian literary scholars and is more often than not looked at as authoritative.
Arthur (Artos, Artorius): It's hard to go into any detail on him without going into LOTS of detail. He's the King. He is rarely an active participant in much plot, and he has some historical basis, but hardly as a high king or emporer. He is sometimes too young/childish/immature/inexperience, sometimes old and feeble. He rarely acts in anger, but often acts hastily. you can't often realistically place a "tragic flaw" on him convincingly.
Igraine (Yguerne, Igrayne): The wife of the Duke of Tintagel, seduced by Uther in disguise as her husband. The issue of her virtue is universally important in Arthurian lit. She is almost always either ignorant of uther's deception, or their relations begin when the Duke is dead (even if they don't know he's dead and it's only a moment or two after his last breath).
Mordred (Modred, Medraut): The Incestuous son of Arthur and his sister. This is our typical understanding, anyway. many time he is simply a nephew. It is his treachery that destroys Camelot. The treachery has a tendency to become more and more severe the later in the history of Arthurian lit you go. Mallory kind of crowns it by having him rape and/or force Guenevere to marry him, AFTER engineering a war between Arthur and his best friend Lancelot, and ending the whole cycle by killing his father/uncle.
Morgan (Morgaine): Often derived or even understood to be a Celtic deity. She is almost always a witch or a at least a pagan priestess. She hates Guenevere more than Arthur, but is certainly an enemy of Arthur. Conflicts surround the engineering of attempted assassinations of Arthur, and stealing Excaliber. In the last century, she is often Mordred's mother.
Morgause (morgawse, Margawse): Igraine's daughter, therefore Arthur's (half-)sister. When Mordred is Arthur's incestuous son, Morgause is *usually* the mother. She is married to King Lot (an opponent of Arthur) and the mother of Gawain, Agravain, Gaheris, and Gareth (3 of which are among Arthur's best knights ever). Mallory makes Morgause and Morgan sisters. often, these two are conflated or confused.
Nimue: Nimue is a largely recent usage as a prominant character, unless you count Vivianne's appearance, who she typically replaces. She's the nymph/sorceress/girl with whom Merlin falls in love. Sometimes it's reciprocated. Other times she just imprisons him in stone. Sometimes Nimue is conflated with the Lady of the Lake.
Pellinore: King of the Isles. He's also often the father of important knights of the round table. He's also involved in the rift between Arthur and Lancelot, killing Lot and being killed by Gawain. He breaks the sword Arthur drew from the stone and Merlin replaced it with Excaliber. He also chases the "Questing Beast" in many stories, starting with Mallory. The Questing Beast, in later stories, is very dragon likes, but it always makes a sound compared to the barking of (questing) of a large number of dogs. The word "questing" is often used in an anachronistic sense to mean "pursuing a quest." I personally like the sense (if I may editorialize) and use it that way.
Uther: Arthur's father. He's almost always blood-thirsty, lusty, and warlike. He either rapes Igraine (in disguise) or contrives to have her husband killed so he can marry her (or both). Sometimes a nominal Christian (if religion is an issue) who ends a long series of feuding, fractious, and destructive Kings finally uniting England (but then throwing it back into chaos when he dies).
