'Lo,
I've been gleefully loitering for a few weeks now, assessing how small a fish i really am!But it's delightful to have such a deep & far ranging pool of knowledge to dive into. I think i'm seriously hooked.. Also i have a silly question- what is the specific definition of a catfish? (as opposed to any other fish.. )
p.s. my bristlenoses just had babies:D, not exactly my achievement, but i'm honoured nonetheless..
Probably something along the lines of 'Well, they all have whiskers..'; I've been trying to figure it out myself, but it hasn't clicked yet. I haven't got my head around the technical terms yet either, so simple is good at this stage, but you can set me a challenge if you like! .
OK, here's a compromise of the the layman and scientific definitions (taken from an ichthyology text):
Catfishes are characterized by the following features (not always present):
Fusion, reduction or loss of a number of skull bones, including the maxilla.
Teeth on the roof bones of the mouth (vomer, pterygoid and palatine).
An adipose fin, sometimes with rays or spine.
An unsegmented, spinelike ray at the front of both dorsal and pectoral fins often accompanied by a shorter spine that helps lock the larger spine in the erect position.
Lack of scales, often combined with the presence of bony plates or tubercles.
Small eyes.
One to four pairs of barbels associated with both upper and lower jaws that serve both chemosensory and tactile functions.
There picture is actually more complicated than that and there are a number of characters unique to catfishes in the Weberian apparatus. Let me know if you need a more exact (and more technical) definition.
To keep it simple, loaches lack the spines associated with the dorsal and pectoral fins, have scales (although in many loaches, the scales are very tiny and deeply embedded in the skin so that they look scaleless), lack the teeth on the roof of the mouth, never have adipose fins, and have a maxilla that is not reduced to form a bony support for the barbels (if present).
) 
.