When people talk about chipping an engine,they mean one of three things:
1. Replacing the chip in the ECU (engine computer that controls fuelling, timing, etc.) with a chip programmed with more optimal parameters.
Installing the chip means unscrewing the ECU case, unplugging the original chip, fitting the new chip, and closing up - not for the novice
2. Reprogramming the chip in the ECU - saves opening the ECU box - but needs to be performed by the tuning garage
3. Fitting an extra "plug-in" box between the ECU and the fuel injectors - this is dead easy - just unplug a connector and insert the box.
Pros and Cons
The plug-in box (number 3) does a good job - but it can't adjust all the parameters or react to all the sensor settings - so it's not as optimum as:
Numbers 1 and 2 are basically identical - just different ways to achieve the same effect - and should give you the best power improvement and best fuel savings.
However, the plug-in systems are perfectly adequate for what you want to achieve.
You do need to have a look at the quality of the plug-in hardware:
The better quality plug-in boxes have dedicated plugs for your car hardwired into the box, and you order a dedicated system for your car - no loose cables - no unnecessary connectors.
The lower quality units use a plug / socket on the box with a loose cable that you attach - this lets them make one box and supply a different cable for different cars - and the concern is with the long term reliability of the plug / socket on the box - often just a 15 pin computer plug - and these are not meant for under-bonnet operation.