The ADXL335 sensor measures acceleration at three axes, X, Y and X. But as it is an analog device, it will not report acceleration in terms of G forces, but as voltage. This voltage, for each axis, is sampled by the Arduino to produce a value between 0 and 1023 (on the Arduino Uno).

If your accelerometer is resting on a table, then x and y measurements will be right in the middle of their range, and z will show a downward acceleration value equivalent to 1G. 

Have a look at the lecture titled "Write the sketch for detecting acceleration with the ADXL335", around the 2nd minute (or see the screenshot below). The Arduino with the ADXL335 is sitting on a table. The values that are coming out of it are around 505 for X and Y, and 616 for Z. These are analog values, and the analog input on the Arudino Uno has a range from 0 to 1023. Allowing for calibration and noise, for my particular device, middle of the range is around 505. And 1G is around 615. 

If you follow this exercise (even better, do it), you will learn about these 1G values across all axes.