A training contract is a low paid deal ($1200 per week), with no job security. They are essentially coming to training to try and earn a better deal. Typically they only run from November when training starts until the end of preseason games. Players on training deals can play NRL, but only from Round 11. Although I don't think many teams would maintain any training contracts once the season proper starts. They would just be upgraded to a NSW Cup or NRL contract, or let go.
A development deal (or Supplementary list contract) is for players who have come through the junior system of a club, but have not yet broken into the NSW Cup or NRL scene (played less than 6 games). Allows them job security, and the ability to train with the main squad. Paid $80k a year. Can play NRL from Round 1, and receive a bonus $3k per game.
The Supplementary List has it's own salary cap $650k per year. Each club must have at least 4, maximum 6 on the list.