Oxford English Dictionary: 

Spade: 

"1.  A tool for digging, paring, or cutting ground, turf, etc, now
usually consisting of a flattish rectangular iron blade socketted on a
wooden handle which has a grip or cross-piece at the upper end, the
whole thing being adapted for grasping with both hands while the blade
is pressed into the ground with the foot." 

Shovel: 

"1.  A spade-like implement, consisting of a broad blade of metal or
other material (more or less hollow and often with upturned sides),
attached to a handle and used for raising and removing quantities of
earth, grain, coal or other loose material. (In some dialects the word
is applied to a spade.)" 

I believe that in more common modern parlance the two words have often
become interchangeable, but the distinction between the two tools is of
considerable antiquity, existing in Old English, German, and Frisian. 
Note particularly in the definition of spade, "... a flattish
rectangular iron blade..."  Both words are independent of each other for
a thousand or more years.  Shovelling is related to shovel, but shovel
appears to be earlier, and the relationship like that of hammering to
hammer.  English is fond of turning nouns into verbal forms. 

Mike in Woodland