Sporádes is a collective name for several island groups in the Aegean Sea. It serves merely as a geographical identifier and has little significance in a historical or cultural context.
The northern Sporades are an island group off the eastern coast of mainland Greece. The island chain stretching east from the coast of Magnesia (the southern tip of Pelion to be precise) into the Aegean, of which Skiathos, Skopelos and Alonnisos are the principal islands, forms the main part of this group. The single island of Skyros to the south of that complex and off the east coast of Euboea is generally included in this group.
The description of the southern Sporades is less agreed upon by geographers. While they all agree on the Dodecanese, including Rhodes, being at least part of this island group, many include the large islands off the coast of Asia Minor in it, meaning Sámos, Ikaría, Lesbos and Chíos. Given that Lesbos is actually at the same latitude as the northern Sporades and Chios is distinctly separate from the rest, I would not include either of them in this group, though the others qualify.
As a rule of thumb, I'd classify any larger islands in the the north-western quarter of the Aegean (north of 38° latitude and west of 25° longitude as the northern Sporades and those in the south-eastern quarter (south of 38°) and within about 25 n.m. of the Turkish coast as well as those north-east of Crete (expressly excluding any islands within that range belonging to the Cyclades) as the southern Sporades.