sleeve tattoo (or tattoo sleeve) is a large tattoo, or a collection of smaller tattoos, that has a unified theme, that covers most or all of a person's arm, 
What'd a sleeve tattoo?
sleeve tattoo has a unified theme, whereas an arm covered in tattoos may have many tattoos 
OK why he need for tattoo in the first place?
History of tattooing
Tattooing has been practiced across the globe since at least Neolithic times, as evidenced by mummified preserved skin, ancient art and the archaeological record. Both ancient art and archaeological finds of possible tattoo tools suggest tattooing was practiced by the Upper Paleolithic period in Europe. However, direct evidence for tattooing on mummified human skin extends only to the 4th millennium BC. The oldest discovery of tattooed human skin to date is found on the body of Ötzi the Iceman, dating to between 3370 and 3100 BC. Other tattooed
Sleeve tattoo
A sleeve tattoo (or tattoo sleeve) is a large tattoo, or a collection of smaller tattoos, that has a unified theme, that covers most or all of a person's arm, usually from shoulder to wrist.
There is a difference between an arm covered in tattoos and a sleeve tattoo. A sleeve tattoo has a unified theme, whereas an arm covered in tattoos may have many tattoos of different styles that does not have an overall unity.
The term "sleeve" is a reference