When discussing different rules regarding moves, such as touch move rule, 50-move rule, or what happens if you make an illegal move, we first need to know what a move is.
The basic definition of the chess move is: with the exception of castling and promotion of a pawn, a move is the transfer of a piece from one square to another square that is either vacant or occupied by an opponent’s piece.
When is a move complete? The move is completed when the player presses the clock after executing the move. If no clock is used, then it is when the player's hand has released the piece.
With these definitions in mind, legal moves are those according to the rules of chess.
The touch move rule states that if you touch a piece you have to move it. No exceptions.
This is often hard to follow as we often touch a piece as a reflex only to realize that we will lose our queen or a major piece. This is where you can show your sportsmanship, and owe it up: move with the piece you touched, learn from your mistakes, and promise yourself: always THINK BEFORE you MOVE, and this won't happen again.
What if it is your move, and you touch your opponent's piece before your own piece? In that case IF that piece can be captured, you must capture it.
Illegal move is any move that is not following the rules of chess. When a player makes an illegal move, their opponent has the opportunity and the right to call the TD and request adjustment: the illegal move must be taken back and a correct move should be made, as well as the clock should be adjusted and two minutes should be added to the opponent's clock.
If the players do not use any chess clock, no time adjustment can be done, however, the move still can be corrected, and if legal move is possible with the piece that was touched (and the illegal move was made with), then the move should be made with that piece. Of course, if no legal move is possible with that piece, player is free to choose any piece to make the move with.
One of the most interesting rules in chess is the 50-move rule, which states that if no captures or pawn movements were made in 50 moves, then the game can be declared a draw. Of course, the tricky part is that players must demonstrate this to the TD, thus correct notation is needed. One of the opportunities TDs have during a long draws endgame is to count then move, and IF they count 75 or more, then they can intervene and declare the game a draw. But remember, this is only after 75 moves!
Hope this helps every player to pay more attention to every move and knowing the rules will help you in your future games.