Mining machines are graded in basically 2 ways:
1 - Hashpower. For BTC this is measured in Gigahashes (GHash). There are many MANY machines, but for example, a USB miner might have a hash rate of 2GH. This is what determines how many coins it will earn over time.
2 - Power usage. This is measured in Watts (W) and is a measure of how much electricity your machine pulls from the wall. The higher the number, the more you pay in electricity costs, and often the hotter the hardware runs (meaning a possible extra cost to keep it cool).
So a miner would be looking for the highest hash power for the lowest power usage possible, also taking into account the cost of buying the equipment. It gets complicated
![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
Hope that helps a bit.
Rit.