You know how all of the knights in Ivanhoe have something on their shields, some kind of symbol or sign of their identity? The art of keeping track of what those symbols mean is called "heraldry." In real life, those symbols are usually connected to a particular family line. They tell you something about what that family as a whole is famous for. Ivanhoe's heraldry is a bit more individual, though. We can learn a lot about each knight just by thinking a little bit about what he carries around on his shield and banners.
Two Men on One Horse: Brian de Bois-Guilbert's old shield carried a traditional Templar design: two knights on a single horse. The narrator explains that this shield design is "expressive of the original humility and poverty of the Templars" (8.43). In other words, the Templars used to avoid accumulating wealth or individual fame and would have felt no shame about sharing a horse with another knight. Now however (as in the 12th century), times have changed.
Gare le Corbeau: When he arrives at the tournament at Ashby-de-la-Zouche, Bois-Guilbert is carrying a new shield, which shows a flying raven with a skull in its claws. (Fancy!) It also has the words "gare le corbeau" (8.43), which means "beware the raven." The raven clearly represents Bois-Guilbert himself; it's a personal threat from him to whoever looks at his shield. This symbol is far more individual than Bois-Guilbert's original two-men-on-one-horse shield. By switching to this new shield design, Bois-Guilbert is announcing his pride in his own lethal fighting ability. Since pride is supposed to go against the values of the Knights Templar, we know that Bois-Guilbert isn't too good at obeying the strict vows of his fighting order.
Cave, Adsum: Reginald Front-de-Boeuf's shield has an ox's face on it. Since his name literally means ox-face (hehe), this shield is logical. Like Bois-Guilbert's heraldic symbol, Front-de-Boeuf's shield is personal and individual. It symbolizes his own puffed-up pride in his fighting power. His motto only emphasizes that conceit: "cave, adsum" means, "Beware, I am here." Front-de-Boeuf's shield shows that he's a big bully. Not only is he full of himself, but his shield bears the marks of numerous different fights. The backstory we get for him – that he's so violent that he wound up killing his own father in a drunken argument – only confirms our original impression from his shield that he's an arrogant man who loves conflict.
El Desdichado: When Ivanhoe first appears on the tournament grounds at Ashby, he is disguised as El Desdichado – the Disinherited. (Scott's Spanish isn't 100% accurate, but "disinherited" is what he is trying to get across.) The design on his shield is of a young oak tree pulled up by the roots. Symbolically, Ivanhoe is the young tree that has been yanked out of the ground (figuratively out of his family) by his angry father, Cedric. By the end of the novel, we assume that Ivanhoe has picked a different symbol – after all, by Chapter 44, he is no longer disinherited.
The Knight of the Fetterlock: When the Black Knight comes to the tournament at Ashby, he carries no pattern on his shield at all. He's going for a complete disguise at this point. During the siege at Torquilstone, though, we see something on his shield: "a fetterlock and a shacklebolt azure" (29.23). In shield terms, a "fetterlock" is like a modern handcuff, and a "shacklebolt" is a leg chain for a horse. "Azure" just means "blue." It may not sound too fancy that the Black Knight has a bunch of blue shackles on his shield, but if you check out a picture of heraldic handcuffs, you might be impressed. The Black Knight's choice of chains for his shield makes more sense once we find out that he's actually King Richard I. Having been held captive by the Duke of Austria, King Richard is probably all too familiar with fetterlocks.