1. Finding the right "tips" - Different laptops have different connectors. You need the right tip for your laptop. This is very simply to find. Basically, laptop AC adapters comprise of 2 ends. One end is male power outlet. You use this end to plug into wall power outlet. The other end is called "TIP" which you will plug this tip into the "DC IN" in your laptop.
Your laptop won't work on the wrong tip. You can only use the " matching Tips"
Warning : DO NOT try to force it in.

2. Finding the right "output voltage" - Different laptops were designed to consume power varied by equipments such as CD-Rom, LCD size, CPU maker and CPU speed. You require to use the "matching voltage" . Only use the laptop AC adapters output the matching voltage of your laptops. You can easily locate your laptop voltage. Mostly, voltage is printed on your laptops. You also can check your laptop manual.

3. Find the right "input voltage" - for tdose users who will travel abroad especially to Europe and Asia. You would need laptop AC adapters that compatible with 220-240 voltage also. So it would be wise...and yes...you are!!.... Always look for AC adapters accept 100~240 voltages. 100~240 voltages AC adapters will go with you worldwide. You do not need to carry 2 adapters anymore!!!.
that's easy!!! You now know the basic knowledge to find your laptop AC adapters.
Better yet!!! AC adapter manufacturers are now promoting "universal AC adapters" .
this emerging adapters can be used with multiple devices. They can accept 100~240 input voltages and also adjust the output voltage to match device automatically.