and just because...
Straight off Adobe's own help file:
Marks And Bleeds options
All Marks
Creates all printer marks at once.
Style
Determines the appearance of the marks. You can choose default InDesign marks, or marks from other applications as listed.
Line Weight
Determines the weight of the lines for trim, bleed, and registration marks.
Trim Marks
Places a mark at each corner of the trim area to indicate the PDF trim box boundaries.
Bleed Marks
Places a mark at each corner of the bleed box to indicate the PDF bleed box boundaries. A bleed box defines the amount of extra area to image outside the defined page size.
Registration Marks
Places marks outside the crop area for aligning the different separations in a color document.
Color Bars
Adds a small square of color for each grayscale or process color. Spot colors converted to process colors are represented using process colors. Your service provider uses these marks to adjust ink density on the printing press.
Page Information
Places page information outside the crop area of the page. Page information includes the filename, page number, current date and time, and color separation name.
interestingly enough, they actual say the the trim lines should line up with the trim...
EDIT:
just for fun I turned on "All Marks" and tried a print. the "Bleed Marks" actually printed an 1/8" OUTSIDE of the "trim marks". the "Trim marks" are in line with the crop box. so I have no idea where it is getting is measurements from.