According to FMA Wiki,
In the manga, it is determined that resurrective Human Transmutation is impossible because a soul that has left the mortal coil has passed on into the afterlife and can never be called back by human means. The attempt will cause a rebound due to both the inherent lack of any particular substance able to match a human soul in value and the fact that the initiated transmutation is reaching for an unattainable goal. Of the known attempted Human Transmutations, the rebound has been the "taking" of parts of the initiator's body out of the living world and into the void of the flow.
In the original FMA anime (not FMAB), however,
In the anime, the results of resurrective Human Transmutation are different. The anime declares that it is possible to bring dead humans back into the living world, but upon being revived, they generally become inhuman Homunculi with the physical appearance and memories of the humans they used to be. Using the Philosopher's Stone or a human life as the material to recall the soul, it is possible to bring back a human as a complete human. Al also implies that it may be possible to complete a preexisting homunculus. If this is done,which it theoretically could be with a Philosopher's stone,(allowing one to survive creating a homunculus and to attach a soul to it) it is unknown if the now complete homunculus will be a full human thus able to perform alchemy, age, and die easily, or not.
In the FMA anime,
In the first anime series, the remains of the failed human transmutation that was Trisha takes on a larger role. After the brothers rush away from the scene looking for help, the deformed, living mass that was created manages to crawl away. Patiently waiting not far from the house is Dante, who feeds it the Red Stones necessary to give it a human form. The Homunculus Sloth is thus created, taking the appearance of Trisha Elric.
In volume 11 of the manga,
In vol. 11, Hohenheim speaks to Pinako about the remains, asking if anything of it looked like Trisha, such as hair or eye color. Edward, who was eavesdropping around the corner, freezes and looks horrified as he listens. The next day, after Hohenheim has left, Edward asks Pinako to show him where she buried the remains and help him uncover them. The excavation seems to cause Edward great pain, as he frequently pauses to regain his breath or vomit between digging. When they unearth the remains, Edward discovers that it wasn't their mother that they had "resurrected" that night, as the hair color and bone structure differed from Trisha's.
So, the conclusion is that they created nothing but some organs that represent a human, in a vague humanely shape, but without the presence of any soul. This thing that they created died soon after in the absence of a soul.
In the 2003 anime, FMA, which is not entirely canon, this vague human shape has a future impact as it is recreated into a homunculus by feeding it a philosopher stone.
But, in the canon manga and FMAB anime (2009), no such events happen and the thing is buried.