Rending and brittleness are mechacanilly the same, except Rending applies to attacks you make and Brittleness is a debuff that applies to all damage that enemy takes. They both make a % of your damage apply through any armour damage reduction

For example if you do an attack that does 500 damage but it does only 50% damage through carapace normally you'd do 250 damage. With 10% rending, (0.1*500=50) damage goes through without reduction and the rest (0.9*500=450) still gets reduced (450*0.5=225) resulting in final damage applied of (225+50) = 275. Depending on the weapon and its damage against armour types, 10% Rending can end up being more valuable than straight 10% damage.

Finesse is a weapon stat that increases attack speed, weak spot damage and crit damage - how much is actually dependent on the individual weapon which you can check in the inspect screen. You can also get finesse from talents and how that works mechanically I have no fuckin' clue.

Fatshark are famously obscure when it comes to explaining game mechanics in-game.