Major Allies are quest givers, mentors, or powerful NPCs who's interests are permanently or temporarily aligned with the PCs, such as kings, archmages, high priests, guildmasters, etc. Major allies can provide structure to the campaign, by giving the PCs quests to complete, rewarding them with magic items, or providing them with valuable information to complete a quest.
PCs can develop a certain amount of attachment or loyalty to major allies, seeing it as a critical part of their lives to protect and serve such a person. A major ally will have a defined personality and possibly a detailed backstory, but may not have stats, since it is not his/her role to be in a combat with the PCs.
Minor allies are henchmen, or NPCs that might temporarily join a party to serve a purpose, like extra muscle for the group, or as a "MacGuffin" character that needs to be protected or escorted from one place to another. A minor ally might be that gruff dwarf you met in the tavern, or a princess that needs to be taken to another land to be married off. A minor ally should be defined by stats, and may have defining characteristics like a certain voice or accent, but will not necessarily have a fully fleshed personality or background story.
|Quest givers, mentors, kings, etc.
|Henchmen, temporary party members, MacGuffins
|Uncertain loyalties, possible quest givers
|Provides goods, services and info - at a price
|One job, no name, rarely talks
|Mooks, cannon fodder
Minor Neutrals are characters who provide goods, services, and information, but at a price. For example, the tavern keeper who knows where the best swords are made, or the town guard who knows who has a bounty on their head, or the healing woman who has a lead on a good local quest. Minor neutrals are only barely above extras, and don't usually need a detailed background or stats, though you might want to take notes on personality traits like "grouchy" or "has a high squeaky voice." Keep track of your minor neutrals, and if the PCs keep meeting them, it makes your world seem more "real."
Major Villains are the characters who are the main antagonists of the plot, it is their actions that oppose the PCs and defeating them may be the main goal of your campaign. Put some work in making your major villains, with detailed backstory, personality, and probably at some point you will need stats when the PCs confront them. A memorable major villain will make your campaign something the PCs will want to return to.
Minor Villains are the "mooks" of your campaign, and they may be a set of stats and nothing else, perhaps a henchman to the major villain, or an assassin he sends to attack the PCs, or a tribe of orcs they meet along the way to something more important going on. A minor villain with a distinctive personality or background story is on his way to becoming a major villain.
Extras are the characters the PCs will probably never think about: the bar wench who takes their order at the tavern, the courier they hire to take a message to the king, the coach driver they hire to take them to the next town. They don't talk, they don't usually have names, and if they have stats, they are not very good at anything except what they do for a living. Extras can be elevated to minor status if they get a name or if they start talking to the PCs and becoming more important to the campaign.
You should implement this idea while you are preparing your adventure. But it's never too late to start making memorable NPCs.
When done right, even brief NPC Encounters will add flavor to your campaign.