A plasmid is a small, circular double-stranded extrachromosomal DNA molecule within a cell particularly a bacteria cell that is physically separated from chromosomal DNA and can also replicate independently.
Expression vector is usually a plasmid (or a virus can also be utilized) constructed to allow for gene expression in cells. The vector is used to introduce a gene of interest into a target cell, and can drive the cell's mechanism for protein synthesis to produce the protein encoded by the gene, thus giving rise to the expression of the gene.
This gene of interest is integrated into the plasmid that is cut by endonucleases and then ligated using DNA ligase. Thus the expression differs from the normal plasmid in that it contains an external gene sequence.
