Found a great writeup on abstract classes versus interfaces at the code project. While not every developer or project makes use of those OOP constructs, it's important to be familiar with OOP terms for a few reasons:

• So you can design better systems
• So you can expand your knowledge
• So you can better understand other developers' code
• So you don't look like a n00b at some developer gathering

I feel that developers using Microsoft platforms are sometimes at a disadvantage when it comes to practical experience with OOP concepts. Not all developers, but if people started out coding in ASP or VB, they may not have the years of OO development that someone starting with java/etc would. I place the blame solely on the earlier MS platform and the "build it quick & dirty" ability that RAD toolkits accommodated. I'm not bashing anyone...there are many great coders on all platforms. But do you know how kludgey it was doing OO with VB6, ASP 3? Yup...pretty goofy.

Anyhow, I like simple straightforward writeups that I can print out & read on the train, etc. Hence the above link. Plus a link to CodeBetter's Feb 2005 articles -- there are some articles in there on OO concepts (abstraction, encapsulation, inheritance, and polymorphism) that will help any budding OO buddy.