What is the difference between Contig and Scaffold?

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 paired-end libraries (Roche) or mate-pair libraries (Illumina).

Scaffolds always consist of contigs separated by gaps. These gaps might be identified by "NNNN" in a consensus sequence

Ref: http://www.eurofinsdna.com

B. Wajid