ok, newbie to newbie (since I'm only about 2 months into all this). a SERT is a device that hooks to your bikes engine control computer temporarily to change the way your engine runs.
it goes between another computer and your bikes computer and can flash a new MAP onto your bikes computer and has the ability to allow you to change the air fuel ratio, the idle speed, the timing, the rev limiter, among other things. i have met people with HD's who know none of this but it seems people who frequent this forum would turn down an opportunity to modify their bikes about as often as Lindsey Lohan would turn down an open bar.
The bikes come from the factory with an EPA friendly MAP. MAP is the instructions your computer follows to deliver fuel to your bike and determines how it runs. Unfortunately while this MAP satisfies the EPA it also doesn't allow the motor to achieve its best performance and on top of that causes it to run very lean which results in hotter operating temperatures than it would with a richer AFR. An already hot running motor overheats more quickly and certainly can't add to the life of the motor.
If you decide to change anything which it seems every one does, and that change is performance oriented as in pipes or an air cleaner, you end up leaning it out even more because its breathing better. It is widely held that the Stage one download is a waste of money and so is getting your bike tuned at the dealer since they are kindof hogtied into staying in good with the EPA. The download increases your max RPM and this is also a dubious thing as your motor is already making its best torque with the stock cams at a lower PRM. There is also a product called V&H fuelpack but it is also not a favorite as its benefits are limited.
That leaves you with several tuning devices, of which I have become at least a little familiar with a few. The SERT, the Super Sert, the Power Commander, the Direct Link, and the TTS Mastertune. SERT and TTS are kinda the same, built by the same guy, with the TTS being newer and having more features. The Super SERT is made by a different company but took the SERT name, probably having to do with the input of HD themselves since they sell it and install it. The Direct Link is very much like a SERT only cheaper. All of these hook to your bike and flash the ECM with a new MAP after a technician gets it all set up after about 50 dyno runs. They have to do so many to get it all dialed in perfect. They marry to the VIN of your bike so you can only use it again on the same bike.
The Power Commander is different in that it hooks on and stay on, removing it changes you back to your stock MAP. It does not have as fine tuning abilities as the others as it hasn't broken the range of your engines operation down into as many small parts to adjust as the others. People who have them like them alot.
I am happily trailering my bike to Dallas this weekend to have its tune done by Ed from the Dyno Difference. One other thing I gleaned from all the reading i have done in the past 2 months is that your tune is only as good as the person doing it. Of all the ones considered GOOD the closest to me is 250 miles away. If there is one in Houston sorry, I missed you. In a new thread I am sure i will be offering feedback on the change in driveability after the tune. For now it is snarling and popping like a dragon after i put new pipes on last weekend.
Thus ends my very basic and rudimentary knowledge of all things tuning. If any of you more experinced folk see errors, kindly steer the conversation in the correct direction.