During a genome assembly, "contiguous sequences of nucleotide bases" (contigs) are built from the multi-alignment of highly similar single reads.
After the alignment step, multiple consensus sequences of all aligned or assembled reads are obtained which represent the contig sequences of a given genome or assembly. In contrast, a scaffold is an ordered set of contigs which are linked by sequences that were derived from the paired-end information of long jumping distance libraries or mate-pair libraries.
Scaffolds always consist of contigs separated by gaps. These gaps might be identified by "NNNN" in a consensus sequence.