Hit Dice means both the type of die you roll to determine hp at level up, and how many of them you have (racial hit dice + class level hit dice)
This is because Racial Hit Dice means that for some (usually Monstrous, non-standard) races, your total Hit Dice is not the same as your Class or Character Level.
Hit Dice - can be Racial or from Class levels. Gives varying skillpoints depending on type, varying rates of BAB, varying rates of base saving throws, and determines the dice size for rolling hitpoints
Class Level - your level in a specific class. Each level in a class adds a hit dice, plus possibly class abilities (like bonus feats, or spells, or sneak attack).
Character Level - A combination of all your class levels (different from class level because Multi-classing can lead to different levels in multiple classes).
Effective Character Level (ECL) - A combination of all your racial hit dice, levels in classes, and LA (Level Adjustment) to get a single number. This is important for determining experience levels and what 'total level' the character is. The idea is that races like Ogre etc that are great cost a number of racial hit dice and level adjustments compared to 'regular' races like Human, but need to be in the same party as Humans etc, so even though they only have 5 levels in classes compared to the 11 levels the Human has (as they spent 6 levels on Level Adjustment and Racial Hit Dice to play their race), they still count as a level 11 character for experience points etc.
Max skill ranks are based on total hit dice, so unlike ECL (Effective Character Level), it doesn't count LAs (Level Adjustments) in that total. So a Level 11 (ECL 11) character with 4 LA (from being a Half Vampire or something with 4 Level Adjustment, I don't know what has that), would only be able to put 7 ranks into skills (11-4). Whereas a character with ECL 11 made entirely out of levels in classes would get to put 11 skill rank into skills. Or one with ECL 11 made out of class levels and Racial Hit Dice.