I'll try to give you a hand with the Ohms definition. Often referred to as Omh's Law, named after its descoverer, Georg Ohm. Ohms is a measure of a device's resistance to the flow of a signal (voltage and current). In an AC signal, this is called impedance. In a DC signal, it is call resistance.
Ohm's Law: E=Volts, R=Resistance (in Ohms), I=current, P=Power
Visually - A cheater way of combining the formulas follows:
E
------- and also P=E*I
R | I
In the layout on the left, to find the value you are looking for, just cover that one up and you are left with the formula. For example, if we want to find current, cover up the I and you are left with E divided by R (I=E/R). E= R*I, R=E/I. The symbol for Ohms is the letter Omega from the Greek alphabet (sorry, this site will not display the character correctly). Multipliers are sometimes used in addition like k= 1,000. So, 10k ohms is 10,000 ohms.
Now this is kind of a generalization - pickups can vary by type of wire used, potting, etc. But for the most part:
In a guitar amp signal, the current stays constant. If we increase the resistance, it results in a higher voltage (E=R*I). In the second formula for power P=E*I, if voltage goes up, so does power. Higher resistance means higher voltage means higher power means higher output. So, pickups with low resistance (7k-9k ohms) are lower output and are more mellow, clean and warm compared to a high resistance, high output (10k-20k omhs+) pickup. Most pickups placed near the neck are low resistance. Don't put a real high output pickup there or your guitar will sound like crap! Save those for the bridge position.
Most Humbucker pickups are offered in matched sets, one for the neck, one for the bridge (don't get them mixed up). Even if you are buying a matched set of "MegaEarbleeders", you'll find the neck pickup has a lower resistance. Single coil pickups are offered in threes (for Strat type guitars).
I thought the ESP EC-50 was a solid body (Agathis) guitar. Agathis is not a very good tone wood. If your guitar is a hollow body, you may want to stear clear of the higher output pickups. It could overpower your guitar and lead to some serious feedback problems. The EC-400's and 1000's come with active pickups, which are very high output. Probably to make up for the lack of tone from the body wood.
I once thought a "Sweeping Arpeggio" was an Italian janitor.