Ok I have read this class some, and played beside them. What in the world are they? They hang around with some outsider all the time and.....Someone please give me a run down on what these guys do because I cannot quite wrap my brain around them. Sorry but I read the entry for it and its like reading stereo instructions for me.
What does this outsider thing look like? is it around all the time? does it have crazy powers? does it meld with you and you get powers?
Read the section on the eidolon to learn more about the outsider they summon. But really, even without the eidolon the summoner class is very powerful. They're a very competent spell caster that either has more summon monster spells as a spell like ability than you can shake a stick at or else they're a caster running around with what amounts to a fighter as their body guard.
A summoner is an arcane caster who, like the sorcerer, has an innate talent for magic. Unlike the sorcerer, the summoner's natural talent revolves around contacting and befriending(or enslaving, depending on alignment?) creatures on different planes.
Summoning comes naturally to the summoner; he can use his Summon Monster spell-like ability to call extraplanar creatures to his side quicker, earlier and for longer than a Sorcerer or Wizard could with their spells. In exchange, a Summoner's spellcasting is diminished; they get fewer spells than a Sorcerer of the same level would.
The summoner has a close bond with one particular extraplanar creature, which is known as his Eidolon(this is the crazy outsider thing you mentioned). This Eidolon can be almost any type of creature a summoner could imagine. The Eidolon can be with the summoner any time the summoner is not unconscious or asleep, but when the eidolon is out, the summoner can not summon using his Summon Monster spell-like ability.
The Synthesist is a special type of summoner who, rather than calling an Eidolon as a companion, fuses with an extraplanar creature to gain some of their abilities.
I'm playing a master summoner right now in a Kingmaker game and I'm finding the class to be complex and rewarding. It does have a distinct learning curve, though.
A plain vanilla summoner can be explained thusly: you have a permanent companion. This companion is completely under your control and you can design it to look and act pretty much any way you want it to. Your spells fall under three basic types. Buffs (for everyone), heals (only for your companion), and battlefield control. You also have a 3/4 BAB and wear armor, so you're not useless in combat either. At higher levels a summoner can merge with or duplicate his / her eidolon.
My character's eidolon currently looks like a tall, raven-haired woman in leather armor. She carries and uses a great axe in combat (for the moment at least, with the master summoner archetype she / it won't stay viable in combat for long - unlike a regular summoner's eidolon who will continue to grow in power).
A synthesist summoner gives up an independent companion and instead wears their eidolon like a suit.
A master summoner gives up most of their eidolon's abilities in order to overload the map with summoned monsters. "Me and what army? THIS army!"
Ultimately, a summoner is never alone. Unless they're asleep ...
And for that reason you always choose Half-Elf as your race when making a Summoner. The extra evolution points and Summoner making good use of Eldritch Heritage (which skill focus qualifies you for) are just gravy, and make it one of the very few builds where Human isn't automatically superior.