What's the difference between POV characters and other characters in the A Song of Ice and Fire book series, given that every character is written in the 3rd person?
The POV characters in A Song of Ice and Fire are the ones whose names appear as the titles of chapters. It's true that the chapters are written from "third person point of view", but this "narrator" only has access to what the POV character has access to. We get to see what they see, hear what they hear, and read their thoughts, internal monologue, their dreams, etc. In a way, they are the ones telling the story. Other characters get to keep secrets from the reader, these don't.
The use of POV is also important because the characters are fallible. For example, one character (circa third book I think) definently misremembers an event she experienced, and we read, from the second book. On another occasion the same event is described by two different people in two different chapters. Only after the second description of the event do you really understand what's going on.
You're asking about what is a point of view in litterature. Reading Narrative mode : Third-person subjective might help you, but be sure to read the whole page if you're interested in the different narrative modes.
The third-person subjective is when the narrator conveys the thoughts, feelings, opinions, etc. of one or more characters. If it is just one character, it can be termed third-person limited, in which the reader is "limited" to the thoughts of some particular character (often the protagonist) as in the first-person mode, except still giving personal descriptions using "he", "she", "it", and "they", but not "I." This is almost always the main character—e.g., Gabriel in Joyce's The Dead, Nathaniel Hawthorne's Young Goodman Brown, or the elderly fisherman in Hemingway's The Old Man and the Sea. Certain third-person omniscient modes are also classifiable as "third person, subjective" modes that switch between the thoughts, feelings, etc. of all the characters.
This style, in both its limited and omniscient variants, became the most popular narrative perspective during the 20th century. In contrast to the broad, sweeping perspectives seen in many 19th-century novels, third-person subjective is sometimes called the "over the shoulder" perspective; the narrator only describes events perceived and information known by a character. At its narrowest and most subjective scope, the story reads as though the viewpoint character were narrating it; dramatically this is very similar to the first person, in that it allows in-depth revelation of the protagonist's personality, but it uses third-person grammar. Some writers will shift perspective from one viewpoint character to another.
The focal character, protagonist, antagonist, or some other character's thoughts are revealed through the narrator. The reader learns the events of the narrative through the perceptions of the chosen character.
The differences lay in the relation between the reader and the characters. It's easier for the reader to feel closer, to identify with the POV characters because he understands their thoughts in a better way.
Also, the POV characters are generally the most important characters (although I've always wanted a Robb POV, which never happens).