These two terms are often used to mean the same thing.But they are not.
One can not do Dhyana , It happens .What mostly one can do is to long for Dhyana. The longing definitely ends in Dhyana someday.
Whereas One can do Meditation . It simply means Practicing to concentrate.
We have a compulsion to express everything in English , it is not possible always.
English is a sweet language , but it is very superficial.
Moreover we can not expect for English to have a proper word of Dhyana when a culture is lived superficially and thus superficial experience of
living contributed to the language making.
Over many decades , most people of India did not paid any attention to their own culture.Now we are living in a time , when in large number Indians are interested in their culture.As it seems , as a country our Prabdha is coming to an end!
Now come down to the meaning -
Meditation - Manah-Sanjog , concentration of mind. If it is not there , human life is a mess.Meditation is very help full to clear out stress , anxiety etc. to clean body , mind and to recharge vitality for very effective and efficient way of living.
There is deeper aspect of Meditation too, which related to concentration on self , watching the mind itself , This gives us a gimps of what we actually are. Contemplate on self , One realizes , there is something more ......
So its just a process to enlarge human experiences within the boundary of Mind.
Dhyana - It is beyond Mind.Speaking about something that is beyond mind is ridiculous. Meditation is just the beginning , when mind settles down , like the dust particle in a glass of water , Atman reflected itself in Chitta , How can anyone describe something that is outside of mind's
reach! Only examples can be given , still they are not perfect since all the examples are material, i.e have a boundary of logical understanding , where as Atman do not have such boundary.
Dhyana can not happen on the roads , while walking , while cycling But Meditation can be done in all these situations in form of simple mind concentration. But one must practice Meditation regularly to nurture mind and breathing so that Dhyana Can Happen !
Intoxicate your brains with breathing (pranayama) ... Keep calm ... watch every breath... all the way down , all the way up .... watching breathing will make your mind to concentrate in breathing automatically.... Breathing will be slower with time .... Kumbhaka (internal or External)
Time will increase .... feel ... your breathing becoming so subtle , it almost do not exits , even a feather will not move if it is placed against your nostrils.... The black vision will change color to red or yellow..... the change will be faster .... It is still in the mind ....
Practicing this will lead to purna kumbhaka ... no breathing for some minute ... and the dhyana happens ... mind DIES.
Oh! What is this ! ---- slightest of thought and dhyana breaks.But the experience of fraction of a minute leaves trail for life time , and that one
Longs for rest of the life to stay in that bliss ... Atman.
Restlessness comes , how to get it ... no where to go , no shop to buy , no god to worship , the treasure of all treasures ........ is within..
All one have to do is SIT IN SILENCE.
Om Namaha Shivaya