Is the king a major piece?

No. The king is in a class of its own: it is never traded, never captured and carries no point value, so it sits outside both lists. It does fight, and in an endgame it is worth about as much as a minor piece as an attacker, but you will not hear anyone call it major or minor.

Why are they called heavy pieces?

Heavy piece is the older term for the rooks and the queen, and it survives in phrases like heavy piece endgame, meaning queens and rooks with no minors left. It comes from the same idea as major: these are the pieces that mate on their own, so they do the heavy work.

Minor pieces and major pieces explained

Bishop and knight: minor. Rook and queen: major, or heavy, whichever your book says. Pawns are pawns, and the king is counted apart from everything.

That is the whole vocabulary, and you need it because half of chess writing assumes it. “Trade the minor pieces” means swap the bishops and knights. “A heavy piece ending” means queens and rooks are all that is left. “He has three minors for the queen” means bishops and knights adding up to nine.

The test behind the split

Put a rook and a king against a bare king and you can force mate, every time, from any starting square. Put a bishop and a king against a bare king and you cannot, no matter how well you play: the position is a dead draw. That is the real dividing line, and the point values follow it.

A queen does it in ten moves at most, which is the first ending anybody should learn: king and queen against king. A rook does it more slowly. A bishop or a knight alone does not do it at all, and even two knights cannot force it.

What they look like on a board

Major pieces on the open d-file: White's queen on d2 behind the rook on d1, Black's rook on d8. The minors are the pieces standing on squares: knights f3 and f6, bishops g2 and e6.

Look at where each kind of piece wants to be. The major pieces want a file. Queen on d2, rook on d1, one behind the other, and the whole d-file belongs to them. Nothing about that arrangement is accidental: rooks and queens work along lines, so they need a line that is not blocked by pawns.

The minor pieces want a square. The knight on f3 covers d4, e5 and g5 and cannot be attacked by a pawn; the bishop on g2 sits on a long diagonal. A minor piece on a good square can be worth more than a rook stuck on b1 with nothing to do.

Where the words earn their keep

Two places, mostly. The first is counting a trade: rook for bishop or knight is a major for a minor, two points, and it has its own name, the exchange. The second is planning an endgame. Trading into a rook ending and trading into a knight ending are different decisions with the same material count, because the rook wins pawn races and the knight does not.

And when someone tells you that a bishop is better than a knight, remember they are both minor pieces and worth the same three points on paper. Which one is actually better depends on whether the pawns are locked, which is a question about the position rather than about the pieces.