Okey I want to know about these pins.
SDA, SCL , A0, A1, and A2.
What are they used for?
sda and scl
i think that sda is the serial dataline and scl is the serial clock. the pins labelled A are the address input pins.
the data sheet for this chip is here http://ecee.colorado.edu/~mcclurel/Philips_I2C_IO_Expander_PCF8574_4.pdf
http://www.i2cchip.com/pcf8574.html
23 pages of datasheet gloriousness, and all it took was “PCF8574” straight into google.
If you don’t feel like typing all those fiddly letters into google, you can just double-click on the “PCF8574” bit and select “copy”, then go to google and select “paste”, that might cut down on the effort involved.
SCL and SDA are your I2C
SCL and SDA are your I2C control pins. They should be wired to SCL and SDA on your micro and have 10k-22k pull up resistors.
A0, A1 and A2 are your address pins, you can wire them to 5v or GND. The address is 0100(A2)(A1)(A0). So if you connect all address pins to GND then your address will be 0100000 and all to 5v is 0100111.You DID read the datasheet, RIGHT?! 
**No they arent for**<br><p>No they arent for controlling it, just for setting its address. Remote 8-bit I/O expander for I2C-bus is written in the title of the datasheet, dont get this if you cant use it.
Are you sure you can`t use I2C? Theres a lot of stuff google throws up for "basic stamp 2 I2C".
"holnomic"
I had to look that one up - and I STILL don’t know what it means.
oh wait, ,you mean the holonomic wheel, not the funky mathematics… um…