AC: Broadly interpreted, this is the chance to be struck. If you have a high or strong AC, you are not likely to be struck. If you have a low or weak AC, it is very likely that incoming attacks will connect.
HP: This is your durability or how many hits you can take. This either could be an absolute number in a traditional D&D sense or a percentage chance to lose a wound level.
By varying those two factors, you can control the overall durability of a character. Note that both factors are relative to the opposition.
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The TANK has high AC and high HP. He is unlikely to be struck and even if he is struck he can take many blows. This is good for personal survivability, but it can be bad because enemies tend to ignore tanks unless they have a very good reason not to. In too many 1E games, the fighter who is supposed to protect the others is just ignored because he is too hard to kill. Mid level fighters are Tanks because they tend to have excellent defenses and HP pools.
The BARBARIAN has poor AC and many HP. He is easy to hit but can eat a lot of punishment. High level fighters can become Barbarians because the monster's chances of hitting increase faster than the fighter's defenses do. Monsters with multiple attacks are particularly effective against barbarians (claw/claw/bite, for example).
The EGGSHELL has strong AC like a tank but no HP to back it up. Hence, once something penetrates the shell, things get messy really quick. Eggshells can lead to swingy combats. Low level OD&D fighters are Eggshells because they are tough to hit in the first place but can still be one-shotted. Highly buffed clerics can also be eggshells; their defenses are supernaturally high but they still have limited HP.
The SQUISHY is in a bad place. Squishies have weak AC and poor HP. They are likely to die quickly if attacked. Magic Users are traditionally Squishy.
So, interestingly, OD&D fighters go through a life cycle of almost all the types. They begin their careers as Eggshells. They then transition into Tanks in the "sweet spot" mid-game. Finally, towards the end, they become barbarians because their defenses don't scale adequetely with level.
Are some of the types better than others from a design standpoint? I think so. I think the Eggshell is a poor design for a PC. Eggshells lead to swingy combats where the character quickly goes from "just fine" to "bleeding out." Usually, only bigger monsters have a chance to penetrate the eggshell's defenses, but bigger monsters also tend to do more damage which is likely to one-shot the eggshell. The Squishy is in a bad place too, but at least the Squishy knows that he/she is not melee capable and steers clear of danger whenver possible.
Others must agree with me. For example, Gary gave his players extra HD in OD&D games at low levels to get past the Eggshell phase quickly. I think its a common house rule to give characters their level 2 HD at first level or to give them a HP kicker of some sorts. This was implicit in 4E's design philosophy and Hackmaster features it as well.