Stacking is combining two things and getting the effects of both. For example a life steal aura of 15% and a life steal orb of 40% will give you life per hit from BOTH items. Essentially the mechanics are you obtain x*0.15 of the aura and x*0.4 of the orb hp back when you hit something x being damage after armour reduction. However it's just simpler to just say you obtain a grand total of 15+40=55% life steal.
Orbs and buff placers-Attack modifiers
Orbs and buff placers are attack modifiers. They enhance your hero's attacks.
Orbs Effects do not stack with each other. This means that if you have 2 or more Orb Effects, then only 1 will take effect/prority. The Orb effect that takes prority is dependant on which Orb effects you may have at the time.
Buff Placers do not stack with themselves or each other unless specified. For example, Elsydeon's effects will not double even if it procs twice. (However, the duration will be extended)
Orb Effects and Buff Placers will stack with each other.
Do note that different sources that provide identical effects will stack. Taking the example of Life Steal, you can obtain such an effect from Remillia's D, Blood Seeker's Vampiric Aura and the Orb Effect of Blood Coffin/Death Mask. While these effects do not stack with themselves (ie Casting Remillia's D twice won't provide any more lifesteal than it already does, Having 2 Blood Seekers won't let you lifesteal any more than you already do etc.), but they do stack addictively together. (ie 33%+15%+40%=88%)
Summary: Orbs Effects and Buff Placers stack but any 2 Orbs will not and any 2 Buff Placers will not as well. Do note that some items are both Orbs Effects and Buff Placers, which means they can't be stacked with anything else. (IE: Corruption orb)
Bash and Critical
Warcraft 3 has a system called "Pseudo random distribution". Most Warcraft 3 spells that have a percentage chance to proc (ie Criticals, Bashes, Evasion) use this system. However, note that Bashes, Criticals and Evasion can all be replicated by triggers.
How does this affect Bashes/Criticals/Evasion? Using the example of Bash, the more times you fail to Bash the higher the chance to Bash becomes making it not entirely random.
As for Bash stacking. If you have 3 Frostmournes (7% to bash) then it'll stack in this way: On attack, Froustmourne 1 fails to bash(7%), Froustmourne 2 fails to bash(7%) and Frostmourne 3 fails to bash(7%). Think of each source of bash as a "chance". Instead of stacking addictively (7+7+7=21%) it'll stack diminishingly (7% of 7% of 7% to get all 3 Bashes in) IE; 3 different chances to bash. The damage from bash effects on ranged heroes is considered physical damage and if multiple bashes occur, only one of the damage sources will used, while melee heroes damage type is magic and damage can be blocked by magic immunity but in the case of multiple effects happening, all damage sources will be applied. The stun effect however is reversed. Magic immunity will block the stun effect from ranged heroes (like Reimu's basic skill) but melee heroes CANNOT have their bash stun blocked by magic immunity.
The same is true with critical hits, except that when you get more than 1 successful Critical hit, only the last obtained Critical is taken. In the event of items, it will be the last item obtained and can be reset by dropping the higher crit mod item and picking it up again. In the event of skills, the items will most likely have priority UNLESS the skill was first learned AFTER the crit item was obtained. In the event of a basic skill that grants crit, It cannot be given priority ever.
The total chace to have a bash or critical effect occur would be as follows. [1-(1-a)*(1-b)*(1-c)...]
Evasion based on the ladder skill will not stack. (this was actually exploited in Warcraft's Ladder) However, if the evasion is based on triggers (ie Pesudo Evasion), which is done by healing before the damage is done, it will stack with Evasion and other Pesudo Evasions.
However, Pesudo Evasion has a flaw. If your hero would take more damage than their maximum HP (unlikely, but possible), then it fails. (unless explicitly coded otherwise) If your hero is at full health and takes damage, Pesudo Evasion would also fail. (once again, unless coded otherwise) An example of this is Sanae's Innate.