The Soul Catcher is a variation of the spell used to create the Soul Bomb.
With ghosts overrunning the town of Harlan, Kansas and unable to send them back to Hell, the Winchesters contact witch Rowena MacLeod about making a variation of the Soul Bomb to contain the ghosts more permanently. Rowena is unsure due to the difficulty in creating the first one, but agrees to try and manages to create the Soul Catcher, a weaker version of the original Soul Bomb. Rowena begins using it to trap the escaped ghosts, but many escape and try to break the barrier keeping them in. Rowena begins capturing the ghosts, but Francis Tumblety, possessing Arthur Ketch takes the Soul Catcher from her, intending to use it to blow open the barrier. After Dean forces Francis out of Arthur by shooting him with iron bullets, Rowena captures Francis and the remaining ghosts.
After seeing more souls rising from Hell, Rowena warns that the Soul Catcher isn't powerful enough to deal with all of the ghosts.
After Eileen Leahy is revealed to have escaped from Hell and worries about going crazy, Dean suggests using the Soul Catcher to contain her. Sam is dubious about imprisoning Eileen with a bunch of psycho ghosts, but Dean suggests instead making Eileen her own as its better than where Eileen was and where she is going. However, the Soul Catcher proves to be unnecessary as Sam finds a resurrection spell created by Rowena that he is able to use to bring Eileen back to life.
The spell required mixing together an extensive list of ingredients. However, Arthur Ketch was able to shorten the process by at least two steps by electrocuting a mixture of several the ingredients. Once the spell is ready, the souls will be sucked into a crystal with a Latin incantation:
However, unlike the Soul Bomb it was based off of, the Soul Catcher was weaker and could only take a few souls at a time, giving ghosts time to teleport away if they saw it coming. Due to its weaker nature, the Soul Catcher could not hold as many souls as the Soul Bomb which could hold hundreds of thousands.