Pigment and Dye Based Inks - What's the difference?

pigment ink picdye ink pic
What is the difference between pigment based and dye based inks?  Why do I need to know?  Why does it matter?  If you want to understand why some inks flow from your pointed nib differently than others, then this difference is one that you should understand.
Pigment based inks contain...pigments!  A pigment is a dry, powdery substance that when mixed with a liquid (like water) leaves behind color.  The pigment is not necessarily water soluble, it may remain suspended in the liquid.  Gouache mixed with water, Pearl Ex Pigments mixed with water and gum arabic, Fine Tec mica pigments, pan water colors, Dr. Ph. Martin Bleed Proof White, carbon based inks, those are all pigment based.  The pigments are a physical substance, depending on their size, they may or may not flow well from your nib.  Larger pigments will have a harder time flowing from your nib.  Smaller pigments will flow from the nib easily.  Because there are physical components to any ink that you create with pigments, you have some control over how thick or thin your ink can be.  Less water and more pigment = thicker ink.  More water, less pigment = thinner ink.
There are several other variables when using pigment based ink, like the quality of the pigments themselves, and the use of additives, like chalk, (talking gouache here).
Dye based inks do not contain any physical components at all.  Their color is derived from chemicals.  Think of Dr. Ph. Martin's Radiant Concentrated Watercolor and fountain pen inks.  Because the color is from chemical compounds, the color range can be huge. Optical compounds can also be used so the vibrancy can be insane.  Depending on the color, they can also be pretty sheer, (due to the lack of physical opaque pigments).  That sheerness is a very pretty quality of dye based inks.  Dr. Ph. Martin's Concentrated Watercolors are sooo pretty.  If you have ever tried them in your nib, you know that they are pure liquid, there is no physical component to slow down the flow.  They may just run out of your nib, depending on how flexible your nib is.  Many people ask if adding gum arabic will thicken dye based ink.  In my experiments and use, this can be moderately successful, depending on the brand, type, and color.  Within the Dr. Ph. Martin's Radiant range, I found that each color is different, as each contains different dyes.  Same goes for fountain pen inks.  I use Pilot Iroshizuku fountain pen ink and really like it, particularly the gray and dark blue (Kiri-Same and Shin-Kai).  I do not add any gum arabic to them.  Because they are dye based, I know that the flow is going to be pretty fast, so I use a moderately flexible to stiff nib.
Knowing your tools and how they work allows you to make informed decisions...like what inks will work well with which nibs.
Conduct your own testing to determine what works for you...the learning is in the doing!