Don't listen to these people, they are bamboozling you because of politics...
The difference between as assassin or general is minions and meta hits.
-All assassins have a meta hit skill (will kill regular Minotaur, Archer or Priest in one hit regardless of hit points)
-All generals have a minion skill, and can purchase minion idols.
Additionally, assassins and generals have a special set of favor items exclusive to each type.
Unfortunately, the meta game is completely busted in Demigod so the above is mostly never realized, so the real things you need to know are
1) Life stacking and items (banded, scaled, unbreakable, Vlemish, Nimoth, scaled -> Plenor) (Tah dah, you just learned 90% of itemization in demigod)
2) Maximizing skill damage (look for builds in the forums)
3) Not dying (knowing when to withdraw, and not getting stuck overextended in a 1 + X on X)
Then team play
4) You cannot function in a team until you can do the above things. At which point you need to learn when to chase, withdraw, or teleport based on team needs. That comes with experience, and also works better when you play with a regular set of people.
Why learn these demigods first? Because they make the above easier and more obvious (so learn these 3 in any order you want, the better you are at the above 4 things, the better these demigods will be for you)
-Good damage + snare on a fast ranged interrupt: Penitence
-Hard to die with a little practice: Shield III is 6 seconds of invulnerability, enough time to teleport out when you are about to die
-Team based damage nova: Surge of Faith: Speeds up allies, and when coupled with Divine Justice, also recovers health/mana killing creeps
2) Lord Erebus
- Good Damage + Heal + snare in bite
- Escape death with bat swarm or mist
- Mass stun to daze enemies for 4 seconds
3) Unclean Beast
-Insane damage output
-Ooze makes it harder to get killed when you are ganked because it slows auto-attacks alot
-Grasp allows chain stuns
After that, things require more skill to execute properly (probably best to learn in this order)
4) Rook, very strong on a good team, but easy to get ganked on a weak one. Rook requires a good deal of patience, and an ability to read the "plays" as they are happening. So get some "not getting caught in a gank" experience 1st
5) Fire torch bearer, very good burst damage, and is exceptionally fast. Also sort of fragile. You need to know lane mechanics well, how to wear someone down and when you can confront them.
6) Occulus: Incredible Spam firepower and no interrupt means you need to be bang on in mana management and late game teamplay.
7) Sedna, team support via strong healing and anti-healing and 2 skill blockers in Pounce and Silence: You need to know how the other main demigods work, because you'll probably be paired with one (Oak, Rook, UB and LE)
8) Frost TB: Good snares, good disrupting of enemy skills and can freeze towers... but you need to understand how to manage life and mana, and you are a little underpowered in the damage output department
9) Queen of Thorns: She has incredible awesome white damage potential thanks to ground spikes, has a very strong start and can be a real pain to kill when you cycle your modes correctly, but, you need to be an expert on when to fire her skills, lane mechanics, and what mode to be in... she is very unforgiving to any mistakes because she has no interrupt, and rapidly loses power after level 10. Not only do you need to know how to play QoT, but the people on your team need to know how to play with her. This is problematic for beginners.
10) Regulus; Cross lane firepower, excellent snares, and explodey goodness with mines. Unfortunately your damage output is on the low side, you are fairly easy to push around, and you have no interrupt. You really need to be a team player and be able to read everything that is happening in the game. You cannot be good with Reg without understanding how the game works, yet you can think you are good with Reg if the other people you play against do not understand how the game works. (pretty much a n00b trap)
11) Demon Assassin: Swap is all around gankaliscious and warp strike is one of the best skills in the game. But you need to be able to float to get x + 1 on X advantages because demon assassin cannot contest in a straight up even fight. You need to know how every demigod works to be good with DA,
12) Hybrid Torch Bearer: When you know everything about demigod, you can become a nasty little pain in the ass who has good burst damage and disruption potential, but you need to have the timings and management down, or you are going to look real stupid and be ineffective.