Can someone thoroughly explain the differences between a character, player, and humanoid?

The Player is an Instance that is created in game.Players. It represents the player (the client, I believe) and its where the player's LocalScripts, GUIs and client-related stuff is kept.

The character is the physical representation of the player's avatar. This is a model in the workspace which holds character scripts, such as the Animate or the Sound scripts. The character is what the player controls and what actually interacts with the virtual enviroment.

Humanoids are Instances that can be placed inside NPCs (and of course Characters) to give them humanoid traits. For example, an object with a Humanoid will have a name tag, as well as a health bar (if enabled). Humanoids can also be used to animate the Character/NPC, and they basically control the functions of a character or NPC or whatever you put them into to make those things look much more... "humanoid".

I'm not a roblox proffesional and I can't say what I wrote above is 100% accurate, but I'm 99(.9999)% sure it is. I you want more information about each, as Fragmentation123 said in his answer, you can go to the following website articles:

http://wiki.roblox.com/index.php?title=API:Class/Player

http://wiki.roblox.com/index.php?title=API:Class/Player/Character

http://wiki.roblox.com/index.php?title=API:Class/Humanoid

Character: A character is a model which contains: Head,Torso,Right Arm,Left Arm,Right Leg,Left Leg and most importantly a Humanoid , it is found in the workspace, and it can be accessed with: game.Players.LocalPlayer.Character , for more information: Wiki/Character

Player: A playeris an Instance which appears when a user joins the game, it appears under the Players service, for more information: Wiki/Player

Humanoid: A humanoid is the "controller" for a character, it holds walkspeed jumppower health etc, for more information: Wiki/Humanoid