In recent decades, we have witnessed an exponential increase in the synthesis and applicability of metal-organic frameworks (MOFs). Simply put into words, these are combinations of inorganic and organic units forming a reticular arrangement. Today, around 20,000 MOFs have been reported in the literature, and out of these, a large proportion finds important and interesting industrial applications. This introductory chapter aims to provide the readers with a flavor of the basics of MOFs, their structure-functional relationship, and examples of some widely studied MOFs. Fundamental understanding of the basic units like metal nodes, organic linkers with different denticity, and secondary building units (SBUs) help in further understanding the reticular organization of this unique class of porous materials. Metal nodes and carboxylate linkers in SBUs have been known to create a metal-oxo cluster, which provides further thermal stability and flexibility. Interesting application-oriented characteristics like surface area, porosity, gas-purification, and separation have been discussed about some well-known MOFs.