A selectable marker is a reporter gene introduced into a cell along with a gene insert. What this means is, the experimenter can tell the right gene is in the cell because the marker can be seen or detected.
Selectable markers are often antibiotic resistance genes; bacteria that have been subjected to a procedure to introduce foreign DNA are grown on a medium containing an antibiotic. The antibiotic knocks out cells which do not have the resistant marker. Those bacterial colonies that can grow have successfully taken up and expressed the introduced genetic material.
An alternative to a selectable marker is a screenable marker, which allows the researcher to distinguish between wanted and unwanted cells.
Examples of selectable markers include:
- the Abicr gene 
- Neo gene from Tn5, which confers antibiotic resistance to geneticin
- Mutant FabI gene (mFabI) from E. coli genome, which confers triclosan resistance to the host. 
- Edited By C. Arias, B.S., C.P.T.
- Callmigration.org: Gene targeting
- Jang, Chuan-Wei; Magnuson, Terry (20 February 2013). "A Novel Selection Marker for Efficient DNA Cloning and Recombineering in E. coli". PLoS ONE 8 (2): e57075. doi:10.1371/journal.pone.0057075. PMID 23437314. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0057075.