The above answers are pretty good, so I'll just quickly reiterate a few things as there were a few pieces of misinformation above.
Souls are indeed functionally a character slot, except incorporating two major features.
First, your soul has an identity. On a few screenshots we can see that souls have "true names" that are randomly generated. They also have a color and a (very cool-looking) shape. There have been hints that a Coup de Grace (killing blow) performed on an already unconscious character (no accidental pvp deaths) will result in a sort of darkening of the soul for the killer (not the victim) tainting it forever, if only by a little bit (or a lot, who knows) per kill.
Second, other than identity, your Soul has memory. Past lives provide small skill ramps (more efficient improvement) of skills honed in past lifetimes. As it was mentioned above, some souls may generate with prior past lives, but not many.
I can confirm as it was said above that a Soul Pack comes with 3 Souls to choose from, and the possibility of Soul Packs being sold closer to Exposition & Launch is pretty high.
One important thing to note is that when a Spark of Life is used and a Soul is bound to either a Ward (longer life, no family) a Traditional Character (medium life + family) or a Non-Traditional Character (short life, inhabit existing NPC's body) a Soul cannot inhabit a new body until their current one permadies (lifespan - CdGs × Fame) and becomes a non-bound Soul.
This next part needs to be very clear:
Talents and Destiny are not bound to a Soul!
While a Destiny may be the result of many things (soul name, soul alignment, birth date, birth location and more) you get a new Destiny (to pursue or ignore) for each lifespan.
A Talent has a tiny chance (I think someone mentioned .5% or 1 in 200 players but possibly lower) of Bodies will have a dormant Talent, waiting to be discovered by a trigger event (such as drowning or falling a certain distance etc) making the number of people with discovered Talents very, very small.
It's worth noting that these Talents are probably not bound to a Soul but to the mortal coil it inhabits, making Talents a one-life boon most are unlikely to interact with but will probably hear stories about.
I hope this clarified some things on the differences between Souls, Soul Packs, Bodies, Talents, Destinies, and Sparks of Life.
TLDR: Soul + Spark + Body = Life
Destiny and a possible Talent are bound (or generated) to a Body, not a Soul as some mentioned above.