A Shardholder is an entity who has "agreed" to embrace a Shard (potentially in effort to "control" it) or otherwise "dispense" its effects. Their personalities have all been changed from those which they originally were due to which Shard each of them took up. However, the original sixteen Shardholders all knew one another.
When an entity takes up a Shard, they take up its power and its Intent. They keep their mind, but the Shard's Intent bends their mind over long periods of time. For example, Ati - who held Ruin - was originally "a kind and generous man", but over thousands of years the Intent of his Shard twisted his mind, driving him to destroy.
This is why Preservation/Leras needed Vin to destroy Ruin. Leras had held Preservation for so long that he was incapable of any destruction; even that of his enemy. Instead, he trapped Ruin, and gave his power up so that Vin (who only held Preservation for moments) could do the deed for him. She didn't hold Preservation for very long, and was still capable of destruction.
This is another reason that Odium is so frightening. Not only is Odium "the most frightening and terrible of all the Shards" (according to Hoid), but the man who took up the Shard (i.e., Rayse) was already a terrible person. That's a very dangerous combination.
The person (whomever s/he is) that holds Cultivation IS, in fact, Cultivation. Assuming that person has been alive with that power for thousands of years, s/he would be changed by the Intent of Cultivation; to nurture and grow things needed for the future.
Currently, there are nine Holders who are known, for whom their Shards are known, and for whom their Worlds are known. There is one Shard for which its Holder is not yet known.
|Almighty/Tanavast (deceased)||Honor||Roshar||The Stormlight Archive|
|Rayse||Odium||Roshar||The Stormlight Archive|
|Unknown||Cultivation||Roshar||The Stormlight Archive|
|Ati (deceased), Sazed||Ruin||Scadrial||Mistborn series|
|Leras (deceased), Sazed||Preservation||Scadrial||Mistborn series|
|Uli da||Ambition||Threnody||Shadows for Silence in the Forests of Hell|
Sazed now holds the Shard Harmony, which is Ruin and Preservation joined.
Fabrials are Cultivation's power. Technology fits that Intent perfectly.