There are five major types you'll run into in the game. You may run into a mixture of them.
Mundanes - These are normal humans, with normal characteristic maxima. They do not pay for powers, they only have equipment and normal things it's reasonable for them to have. They are between 0 and 50 points. The mercs you encountered were 25 point normals. They had SMGs, claymores, and some combat skill levels.
Superthugs - These are superhumans who have power, but are generally at a lower level than the party average. Some characters may become Superthugs as the game progresses, as you outstrip them in power level. They generally range from 100 to 300 pts. They are generally weaker than the characters. Snowjob is a Superthug.
Regular Opponents - These are superhumans who have powers and abilities equivalent to your own characters. This power level ranges from about 300 to 450 points. You have a good shot at taking these guys on if you're smart and careful, but bad dice rolls on either side can decide a match. Clerity and Wing-Girl are Regular Opponents. All your instructors were also Regular Opponents (with one exception).
Boss Opponents - These are superhumans who have powers and abilities that are much more powerful than you are. They are tough nuts to crack, but the entire team can bring them down if they are careful and work as a team. This will generally devolve into a slugging match. They generally have power levels in the 450 to 750 range. Bosses can skirt some of the campaign restrictions on defense levels, attack levels, or both. Midnite is a lower level Boss.
S-Class Bosses - These are superhumans who have powers and abilities that far outstrip your characters. It's possible to take down an S-class boss, with luck and teamwork, but expect several people to be knocked out, and chances for PC death come into play. S-Class Bosses have power levels in the 750+ range and generally ignore the campaign restrictions on attack levels and defense. Sanction is an S-Class Boss.
Now, there is also divisions by overall type :
Cannon - A cannon is generally someone who dishes out way more damage than they can take. A Cannon may be a Glass Cannon if they can't take much damage at all, or if they have some fatal weakness. A cannon may be a projector, or a melee character. The important thing is, they are specialized in laying down damage on a target and making it hurt. Aegis is a cannon.
Punching Bag - A punching bag has a lot more defense than offense capability. They can take a punch, take another one, take a shot, take a blast, and crack wise all day long. Punching bags generally don't do as much damage as they can take, their shtick is to slow you down and bog you down while a cannon finishes you off. They are often teamed up with Glass Cannons. Midnite is a Punching Bag (as was evidenced by everyone hitting the hell out of him and him not even budging).
Pro from Dover - A Pro from Dover does one thing, and does it really well. Like shoveling. They aren't much good for anything else, and if their shtick doesn't work, they have nothing to fall back on. However, they are a maestro at their shtick, and perform well above their weight class when using it. Clerity is a Pro from Dover.
All Around Joe - An All around Joe is a well rounded character who doesn't excel at any one thing, but also doesn't have any situation where they don't have at least some option of what to do. An All Around Joe is somewhat hard to play as they don't really shine in any given situation, but they also don't suck at any situation. Whip was an All Around Joe.
Monopoly Mike - A Monopoly Mike isn't necessarily good at anything... other than having a bunch of 'Get out of Jail Free' cards. They may be hard to hold, or they may have an excellent method of getting the hell out of dodge, or they may have some way of getting rescued. This could be power sets that make them hard to keep locked up (Celerity or Aegis for example) or they might have powers that make it easy for them to escape (Snowjob or Aegis for example), or they may have access to powerful patrons (the guy who always gets pulled out by his NSA buddy to do a government job in exchange for release).
People can be mixtures (Celerity is a Pro Monopoly Mike from Dover, Aegis is a Cannon Monopoly Mike).
