In the SMILES language, there are two fundamental types of symbols: atoms and bonds. Using these SMILES symbols, one can specify a molecule's graph (its "nodes" and "edges") and assign "labels" to the components of the graph (that is, say what type of atom each node represents, and what type of bond each edge represents).
The same is true in SMARTS: One uses atomic and bond symbols to specify a graph. However, in SMARTS the labels for the graph's nodes and edges (its "atoms" and "bonds") are extended to include "logical operators" and special atomic and bond symbols; these allow SMARTS atoms and bonds to be more general. For example, the SMARTS atomic symbol [C,N] is an atom that can be aliphatic C or aliphatic N; the SMARTS bond symbol "~" (tilde) matches any bond