Tight junctions are multi-protein complexes that hold cells of a same tissue together and prevent movement of water and water-soluble molecules between cells.  In epithelial cells, they function also to separate the extracellular fluid surrounding their apical and basolateral membranes.  These junctions exist as a continuous band located just below the  apical surface between the membranes of neighboring epithelial cells.  The tight junctions on adjacent cells line up so as to produce a seal between different tissues and body cavities.  For example, the apical surface of gastrointestinal epithelial cells serve as a selective permeable barrier that separates the external environment from the body.  The permeability of these junctions is dependent on a variety of factors including protein makeup of that junction, tissue type and signaling from the cells.
Tight junctions are made up of many different proteins.  The four main transmembrane proteins are occludin, claudin, junctional adhesion molecules (JAMs) and tricellulins. The extracellular domains of these proteins form the tight junction barrier by making homophilic (between proteins of the same kind) and heterophilic interactions (between different types of proteins) with the protein domains on adjacent cells. Their cytoplasmic domains interact with the cell cytoskeleton to anchor them.