This is a nice site with FAQs on fountain pen ink and other pen topics.
It's got a nice breezy style about it too:
If you think that fountain pen geeks are anal about their pens, just wait until you hear them babble on about their inks!Fountain pen image courtesy of Bismac under a Creative Commons license.
To hear most enthusiasts talk, you'd think that ink was some magic substance rained from heaven to be captured in little crystal bottles; like some idol that must be appeased, it loves some pens and disdains others, and in extreme cases can eat through an unsatisfactory pen like that icky green blood stuff in the movie Alien.
Some folks feel they must treat their ink like prescription drugs, throwing them out after a year lest they perform some sort of Jekyll-to-Hyde transformation in the bottle (or in the pen). Still others interpret the brand name as a sort of "blood type," and fear to use Brand A ink in a Brand B pen.
And yet, as a friend of mine (and frequent ink seller) sometimes confesses in unguarded moments, fountain pen ink is basically colored water, perhaps with some detergent or thickener thrown in. If you understand a bit of what inks are made of and how they work, you'll find them less mystifying and more trouble-free and enjoyable to use.