on a UNO ,what are these for ? another icsp connection or what ?
and a four holes with J2 what are they for ?
just curious
They are for programming it if you have a hardware programmer.
knuckles:
on a UNO ,what are these for ? another icsp connection or what ?
and a four holes with J2 what are they for ?
just curious
Arduino Unos are really two Arduinos in one!
If you look closely at the board you'll see a small chip that says "Atmel mega16U2". This chip is programmed to do the USB->serial interface.
I'm guessing the second set of header pins let you reprogram that chip if you want to. Maybe you could use it as a math co-processor if you're not using USB... 
Yes but they are not the ones close to the reset button. On my board those 6 lines are just pads to solder headers onto if you want.
knuckles:
on a UNO ,what are these for ? another icsp connection or what ?
and a four holes with J2 what are they for ?
just curious
It sounds like you have an Uno R3. The reset button was moved between the R2 and the R3. The 6-pin header near the USB port is for ICSProgramming the ATmega16u2. This chip has a bootloader, so it can also be programmed via the USB port. The 4 holes marked J2 connect to various port pins on the 16u2, which can be useful if you ever want to do more than USB-to-serial conversion with this chip. Some time ago I wrote some code which allows the 16u2 to work as an ICSP programmer, meaning you could update the Arduino bootloader without needing a second Arduino. I used one of the J2 pins to switch the software between USB-to-serial mode and ISCP mode.
thanks to all for a great response i was wondering how you would work on the U2 chip ,that gives me tips for further research on this