extents() returns an array of five integers representing the left bearing, right bearing, width, ascent and descent of the string using the font, or the font itself, as appropriate.

The left and right bearing are the distance from the drawing origin to the leftmost and rightmost extent of the drawn string respectively, measured in pixels.

The width is the length of the current string, measured in pixels. This measurement supplies information regarding where to begin drawing the next string, if there is more than one. The returned integer may be greater or smaller than the value of the right bearing.

The ascent and descent are the pixel measurements of the extents of the current string above and below the baseline.

If you're working with unix, all the above should be correct. If you're using win32, the left bearing will always be 0, the right bearing will match the width, the ascent is actually the line-height + 1 pixel, and the descent is the descent + 1 pixel. Use GdkFont properties if you need the ascent and descent values under win32.