the depth of field (DOF) means to put more
'ranges of distances from the camera' into focus.
Telephoto lenses have shallower
DOF than wide-angle lenses, when comparing between image sensors of the same size. (Image sensors are the digital 'film'). Therefore, using the same lens settings on any two brands of lenses will technically give the same DOF. The lens brand doesn't actually matter; its focal length and aperture settings do.
Zooming in further with any lens (which is increasing the focal length) will give a shallower
DOF, which means anything distant from what you're focusing on will be out-of-focus (OOF).
Telephoto lenses are often used for their stronger OOF effects. Such OOF parts of the image are known as 'bokeh'. See http://en.wikipedia.org/wiki/Bokeh
for more useful information on this.
'Equivalency' refers to the conversion of focal lengths to make them the same focal length as on 35mm film size, or whatever is being used as the gauge, because many photographers are used to this film size.
Even when using the same focal length, lenses have different angles of view because of different image sensor sizes.
What do I mean?
Well, it's difficult to explain without confusing you any further, so please visit this very useful website for much more useful information - http://www.cambridgeincolor.com
May I bring your attention to the 'Camera lenses: Understanding focal length and aperture
' and 'Understanding depth of field
I hope i've been of some help