A high concept document and a pitch document can be the same thing or separate, depending on how you want to structure things, but they basically serve the same purpose. A high concept document is typically one page long and summarizes the basic concept and key points of a game. The purpose is to have a short paper that you can hand off to someone (team members, management, publishers, etc) which can give them a quick idea of what a game is about and what's so special about it.
Keeping it short is important. A company boss or a potential publisher isn't going to read a huge design document before making his decision on whether or not to approve a game. They don't have the time. Not to mention that designers don't have time to make a giant document for every unapproved game idea they come up with. The high concept document isn't the place to describe the entire plot, profile the main characters, or explain the details of the battle system, a quick summary of the key points is all that's needed. Some people also like to toss in a short slogan or description type sentence at the beginning. There's many ways you can organize a high concept doc, depending on personal preference and the type of game, but the way I used for Ink makes a pretty good base.

The pen is truly mightier than the sword.

Venue
  Ink is a full length turn-based Japanese style RPG designed for current generation game consoles such as the Playstation 3 and Xbox 360.

Features
  - A large dynamic world to explore.
  - A deep twisting plot which intertwines two stories into one.
  - Hours of quality voice acting.
  - Write to change the world, solve puzzles, power-up characters, and turn battles in your favor.
  - Strategically manage ink.
  - Learn and unleash powerful combination attacks.
  - In-game text written in third-person novel style.
  - Read the in-game novel to review events and uncover secret information.

Premise
  A struggling author is drawn into the world of his unfinished novel. But the story has changed... To find his way home, he must join forces with the intended heroes and use his pen to change the world and find the one who is rewriting his story.

Gameplay
  Join Josiah as he lives out the events in latest novel and tries to make sense of the drastic changes that are occurring in the story he so carefully created. Team up with the heroes of the story to explore a vibrant world and fight and write your way to the truth.
  Engage enemies in fast paced strategic battles with a system reminiscent of those from the best titles in the genre. Collect ink from the essence of fallen foes and use it to rewrite the world. With enough ink, there is little that Josiah can't do, but it must be managed carefully.
  Write to change the world and solve clever puzzles. Write to turn the tide of battle in your favor. Gain inspiration and write to create devastating combination attacks. Write to control the growth and development of your party. Write to complete Josiah's unfinished story and discover the one who is rewriting it to suit his own purposes.