monsters are marked immune if they have 100+ res against some type of dmg. immunities can be broken in 3 ways: some aura, some curses and sunders.
regarding auras: conviction aura works vs immun monsters at 1/5 of effectiveness. if that is enough to bring the resist below 100, the monster is no longer immune. example: monster has 120fr, conviction removes 150/5=30, so monster it has 90fr and now takes fire dmg. sanctuary aura removes physical immunity from undeads since it sets phys res to 0 (with some minor details)
regarding curses: lower resist works basically in the same way as conviction. note that combining lower resist and conviction still makes them work at 1/5 against immune monsters! (even if one breaks the immunity, the monster is still internally marked as immune for the game, so the second effect is still diminished). decrepify and amp work in the same way, just against phys immunes.
regarding sunders: sunders just set the resist of an immune monster to 95%. it is still internally tagged as immune, so auras and curses work at 1/5. the advantage of using a sunder is that auras and curses have a lower starting point to work with. consider example above: with sunder, fr is first set to 95, then conviction removes another 150/5=30, so monsters is left with 65fr (compared to 90fr without sunder). this is quite a noticeable difference!
after these effects, all other direct -res from your gear is considered on top. note that direct -res only works against monsters with resists <100, so you cant break immunities with direct -res (but just with the three options mentioned above)
i hope i got all your questions included here, if not, just reply
D2:R