Minecraft Skygrid servers are using special map called Skygrid, which was designed by Seth Bling. On Skygrid servers blocks are placed in a grid with empty space between then, which means players can fall into void if they're not careful.
Skygrid vs Skyblock?
Unlike Skyblock map on Skygrid you generally start without much or any resources. Basic items like tools, food, armor and building materials can be found within map itself, then there are chests and spawners that would randomly spawn all over the map.
Is Skygrid just another Survival gamemode?
Sort of, gameplay on Skygrid servers is very similar to one on Survival servers with extra twist, since there's always a chance of falling into void and some mateirals are not readily available. Skygrid falls somewhere between Survival and Skyblock servers, but unlike Skyblock map you're not limited to your island on most of Skygrid servers.