Of all the Humans encountered by the SGC throughout the Milky Galaxy, are they the same species, different sub-species or completely different species altogether ? Because I remember one of the characters mentioned that the Goa'uld came to Earth approximately 10'000 years ago. Also I think I remember somethink that the Goa'uld started to breed/create the Jaffa about then.
But if you want a more detailed answer...
The Ancients are the first evolution of humanity, originating from another galaxy before they came here. At some point they seeded Earth with our "generation." It's not said how, if I recall correctly, there is some sort of implication with the Dakara "weapon." Early on in the series, they talk of how humanity evolved on Earth, though whether or not they continue to do so after they learn more about the Ancients I don't know (though if we just popped into existence it would kind of mess with the fossil record).
The Goa'uld call those from Earth "Tau'ri," meaning "first ones" or "those of the first world." The vast majority of humans in the galaxy is from the Goa'uld taking primitive humans, though it's entirely possible the Asgard or others also had a hand in it. However they got there though, they are still humans, with just a couple of extremely minor notes.
Because some Ancients bred with primitive humans on Earth, some have some Ancient DNA. At this point Ancients were "advanced humans," with healing powers and so on as they got close to Ascension and there's a few minor ways it's affected those with the DNA, most notably being able to make a lot of Ancient technology actually work. It's never stated how common this gene is in humans from other Milky Way planets, though the Asgard's special interest in Earth humans may be a hint.
It is implied that there may be more to Jonas Quinn, with Nirrti being very interested in him when she scans him with Ancient technology and his super-learning ability is perhaps a little odd, though as it appears to be unique to him and not his people. Given that, he could just be very intelligent with a great memory. As it is, this is a pretty speculative thing. It would maybe have been explored more if Michael Shanks hadn't come back and Jonas got dropped, but as it is, there's not much to say.
Then there are the Hebridians, who interbreed with the non-human Serrakin. Whether this is able to happen because of a massive coincidence in Serrakin DNA or, more likely, the aid of advanced technology, these people wouldn't be 100% human by definition.
The bounty hunter Aris Boch appears to be a human, but apparently his people have a unique physiology that makes them immune to becoming hosts. Since he is the only one that ever appears, in one episode, we don't know any more than that. According to the Wiki, some novel states that the Ancients "modified" them. Make of that what you will.
Lastly there's the Enkarans. They look very human, but need a very specific climate or they'll go blind and die. SG-1 are able to reunite a group of Enkarans they were trying to resettle with their "homeworld," which notably doesn't have a stargate. Their physical appearance is way too much to be a coincidence, but it's not elaborated upon.
The humans in the Pegasus galaxy are, again, somehow seeded by the Ancients, but they don't say how. There are a couple of occasions where Pegasus humans also have the Ancient Technology Activation (ATA) gene - again implying some interbreeding - but it appears to be far less common than among Earth humans (both times I can think of offhand the characters are royalty of their planets). Teyla notably has a few Wraith genes due to experimentation on her ancestors, but this remnant is apparently very rare and only provides a few abilities (can sense if Wraith are nearby, can fly their Hive Ships, etc).
The humans in the Ori home galaxy - the original home of the Ancients - were likewise seeded by the Ori, though it also isn't elaborated upon.
Lastly, due to time travel duplicates of the Destiny Expedition, a human civilisation exists in at least one of the galaxies that the ship travels though.
In all these cases, apart from the Hebridian-Serrakin offspring and possibly the Enkarans and Aris Boch's people, they are still essentially at least 99% the same as Earth humans and are regarded as such. The Jaffa are the largest, most notable exception to this rule, being artificially created by the Goa'uld thousands of years ago. Although made from humanity, everyone considers them a different species.
The humans in the Milkyway galaxy are all the same species. They are descendants of humans transplanted by the Goa'uld from Earth to other planets. Jaffa are a separate species created artificially from humans by the Goa'uld to incubate their larvae and to serve as their warriors.
The humans in the Pegasus galaxy are descendants of the Ancients, who look remarkably human. Humans and Ancient are presumably different species, because they have lived millions of years apart. However, I don not recall any reasonable explanation of how the Ancients and present-day humans could be so alike.
Jaffa are no longer truly humans. All of the rest of the humans found in milkyway galaxy are Humans from earth that the goa'uld transported to other worlds. So they are all the same species as we are with no more differences then race at this point (aka breeding different dominate traits). The humans in the Pegasuses galaxy are also from the same source, but there was a few million years between the seeding of the milkyway and the seeding of the pegaes galaxy so the humans genetically could be considered a different sub species at this point.