What is the distinction between sparse and dense graphs?

Question

I read it is ideal to represent sparse graphs by adjacency lists and dense graphs by an adjacency matrix. But I would like to understand the main difference between sparse a

Answer 1

Main graph integral characteristics are number of vertices V and number of edges E. The relation of these two determines whether graph is sparse or dense (wiki page here).

The whole theory behind choosing graph in-memory representation is about determining the optimal access time vs memory footprint tradeoff, considering subject domain and usage specifics.

Generally you want to have O(1) access time (and thus store the graph as a dense adjacency matrix) unless you can't tolerate memory footprint, in which case you choose the most appropriate sparse matrix representation (wiki page here).