Spirit is a creature type.
Spirit is a name for a collection of ethereal beings. Beyond that spirits can have any form or function.
Before Kamigawa block
After that came Arabian Nights. The set originally had no spirits but had the ctype tacked on after R&D's big creature type change.
Antiquities only had .
Legends had a grand total of four spirits. Namely: , , and .
From there on more and more spirits were made. In Ice Age a few creatures were given the spirit type even though they were more Elementals by flavor (Such as Flame Spirit) From there on every set had at least a few spirits with the notable exception of Nemesis. When the themes of sets became stronger it was obvious that there was less room for random creature types and there were a lot less per set. Invasion block has two spirits, one in Invasion and one in Apocalypse (set).
As the last silence before the storm there were three spirits thrown into Darksteel
Kamigawa block was centered around the Spirit creature type. There are 70 cards with the creature type spirit and 86 cards that mention the word spirit in Champions of Kamigawa alone. These numbers continued as 45 cards in Betrayers of Kamigawa and 49 cards in Saviors of Kamigawa. Also in Kamigawa block, Arcane spells were featured. These instants and sorceries were basically spirit instants. Soulshift was a mechanic in Kamigawa Block that let you bring back spirits from your graveyard to your hand.
This means that as of writing about half the spirit creatures in Magic are from the Kamigawa block.
After Kamigawa block
Time Spiral block with its strong 'past' theme had 11 spirits, all of them references to older, existing spirits.
Shards of Alara has no spirits.
The Zendikar Block had 4 spirits, , , and .
The Innistrad block, which has a night-type horror theme, has a focus on creatures of the night, each taking up an allied color pair. Spirits took up Blue-White in this block as ghosts and other apparitions. This added a total of 40 new Spirit cards, with additional non-spirit cards summoning 1/1 Spirit tokens, such as and . With additional non-spirit and spirit cards providing additional support with acting as a Spirit Lord, and returning spirits from the graveyard.
After Innistrad block
The Return to Ravnica block had a few spirits like the original Ravnica block, adding 9 spirits including Legendary Spirit .
Battle for Zendikar had 1 spirit, .