spoilage for...well anything up to and including season 4 with the little ones in it
sg-1 first comes across signs of hte asgard on cimmeria way back in season 1 'thor's hammer'. danny's looking for signs that the goa'uld are not the creators of the gates adn, while milling around in various legends, he comes to believe that the gods of norse mythology, the asgard, are aliens not like the goa'uld.
they go to cimmeria, 'thor' talks to them via hologram...long story but they evenutally leave the planet which had been under asgard protection without it's protection device. danny leaves behind a sagan box...which is basically humans 101 for the cimmerians to give to thor should he ever return.
(thor's chariot)
fast forward a year later...the aforementioned sagan box comes careening through the gate and the sgc discovers that the goa'uld have discovered that cimmeria is no longer protected, so being villians that htey are, they invade it.(it's the first time we see heru'ur too)
sg-1 realizes the whole thing is their fault so they go to fix it...in the ocurse of that sam & danny make contact with thor who happens to be...well a roswell gray.
they fess up and thor shows up in the nick of time.
they are left with a 'don't call us, we'll call you when you're older' message.
fifth race
jack accidentally gets the knowledge of the ancients downloaded into his head and is driven to build a...thing that allows the gate to dial an asgard homeworld way off in another galaxy. they remove the information and apparantly like what they see...
fair game
right in the middle of sam's promotion ceremony thor grabs jack...seems the system lords are a little ticked at them for killing off various goa'uld and they want to attack earth. thor offers his services to negotiate a treaty to make earth a protected world...the kicker is if we're protected we can't be off expoling the universe.
nemesis/small victories
thor pops back up and asks jack for his help. seems his ship has a replicator problem and he needs jack's help to save it and earth...sam runs off with thor to save his world while jack & teal'c play centipede with mechanical spiders....
and the asgard are supposed to pop back up in season 5
the big points are: they're roswell grays, have been visiting earth for quite some time, control the goa'uld by basically bluffing them, the goa'uld have no idea just how thinly they're spread, they adopt worlds and protect them from the goa'uld, they get really ticked off if folks steal from their protedted worlds(shades of gray, they were amongst the ones complaining about the renegade sgc)
and they seem to really like jack...