I want t ask that how can I choose the right motor for my projects of course there are a measures to know the power and the torque in a motor .. for example with resistors we can know how much it can resist by the bands and those bands refer to a number as a result we choose the right resistor in our circuit ?
Please I need an Explained answer and I apologize, if my English is terrible 
First you need to know how to measure (or estimate) the torque and speed required by your machine. As you have not told us anything about your project I can't say any more about that.
When you know the required torque and speed you can look at motor manufacturers' datasheets to find a suitable product.
I presume you are aware that you can use gearing to match (say) a high speed low torque motor to a lower speed higher torque machine.
...R
Robin2:
First you need to know how to measure (or estimate) the torque and speed required by your machine. As you have not told us anything about your project I can't say any more about that.When you know the required torque and speed you can look at motor manufacturers' datasheets to find a suitable product.
I presume you are aware that you can use gearing to match (say) a high speed low torque motor to a lower speed higher torque machine.
...R
yes I know how to use the gears but I just wanna to be able to recognize the different between different motors ?
recognize the different between different motors ?
Read the data sheets.
ShehabAldeen:
yes I know how to use the gears but I just wanna to be able to recognize the different between different motors ?
I made suggestions for that as well. There were three parts to Reply #1 and you seem to have focused on the least important one.
...R
Rated power, rated current and rated rpm are the key parameters in the datasheet - these are
usually continuous ratings (unless the datasheet says otherwise). Sometimes you get values
for the best efficiency point (which is somewhat less than the rated power usually).
DC motors have a motor constant, measured in Nm/A or rads/s/V (one is the reciprocal of the
other), which will tell you how torque relates to current and speed relates to voltage. This is
a property of the number of turns in the windings, so a particular motor may have several
variants with different windings (and thus different motor constant), yet are identical mechanically.