But what does it all mean?

[Guitarzan]

Ok, I've read all about ohms on the internet, in books, and I still cannot find what they do. So...what exactly are they, I know they are the resitance, but my main question is what do they do in terms of guitar amps. Basically, what do they do to my speakers? What happens if I go higher or lower on my solid state amp?

I can't think of anything else now but any input is helpful.

[Tellboy]

In terms of guitar amps you normally only need to worry about speaker 'ohms'. Most speakers are usually manufactured either 8 or 16 ohsm - this doesn't mean the 16 has twice the power of an 8  - it just means it's available in different format to match the usual amp outputs.
Valve (tube) amps have to have their output matched to the total speaker load (ohms) and have an output transformer inside usually with 16,8 and sometime 4 ohm settings. You must set this to your TOTAL speaker load - if you have two 16 ohm speaker cabinets connected to an amp in parallel (this is the usual two speaker jack socket on the back of most amps)  you will get a TOTAL load of 8 ohms.
Solid state (transistor) amps are different in that they can usually accept a variety of speaker loads without adjusting but they are usually rated at a certain speaker load i.e. a 100watt SS amp might be rated at 100watts into an 8 ohm speaker load (will be higher rating for 4 ohms, less for 16 ohms). This will be stated in the user manual or contact the manufacture. My son has a Marshall Mode 4 which is rated at 350watts into 8 ohms - thankfully he has just got a single 16 ohm cabinet which ONLy gives around 260 watts (still ******* loud).
All similar 'ohm' speakers are not equal - they have different charactereistic sounds and different power ratings (e.g. you can get an 8 ohm Celestion Alnico Blue speaker which sounds incredible but is horrendously expensive and will only take 15watts and you could get another Celestion 8ohm speaker which could take 100watts, but might sound ***t to you). Also whether they are in a closed or open backed cabinet affects the sound.

With regard to pickup 'ohms' this gives a GENERAL indication of how 'hot' a pickup is wound . Single coil pickups are usually around  6 -7k.(6000 to 7000 ohms).  Humbucker pickups (dual coil) usually start from around 7k and go up to 20k plus. (look at all the product details on this site) However there are other factors which decide pickup output and voicing such as type of magnets used and thickness and type of wire - you will find most of this information in these forums. If you have a specific query you can't find the answer to Tim will be able to help you.

And most important don't forget - "There's no place like 'ohm" :roll:

[Bob Johnson]

A very basic explanation: The Ohm is the name given to a unit used to measure electrical resistance. That is resistance to the flow of electricity in a conductor. The word resistance is used for direct current (DC) where one conductor is always positive and one negative. When the current is alternating (AC) the term Impedance is used. Impedance is fundamentally the same as resistance but calculating it is more complex. Where speakers are concerned the rating in Ohms refers to their impedance.
The higher the numerical value of this rating the greater the impedance (resistance) to the flow of electicity in the speech coil, therefore a higher value will reduce the output of the amplifier. This means there will be a reduction in the current (Amps) flowing through the speaker. Depending on how the amplifier is constructed this can/will cause a voltage rise in some components possibly leading to eventual failure.
A speaker with a lower Ohms rating (impedance) will increase the output of the amplifier by allowing more current (amps) to flow. This can result in over heating and eventual failure.
This is a really simplistic view and there will be lots of exceptions but if you look at say a Peavey Ultra Tube you will see 4, 8 & 16 Ohm outputs where the output transformer has been tapped to cater for different impedances. Others will give you a safe range; my Traynor manual says 4 to 12 Ohms is OK. If you don't have this facility my advice would be to stick with using speakers of the correct impedance.