Alex has a great series of electronics tutorial, he writes –

Welcome to the electronics tutorial! If you want to learn electronics, it’s a great place to start. You can also revise for your GCSE here, as it’s written to GCSE standard for the year 2000 (so if you’re taking it in 1999, forget it. but have a look anyway!). It’s suitable for all, and anyone can learn electronics using a combination of this & the other material available on my site. Whether you want to learn as a hobby, are taking a course in it, or just want to brush up on a few points, there should be something here to help you.

Electronics tutorial – sections  1- 5 & contents – Link.