There are two kinds of communication junctions: gap junctions and plasmodesmata. Gap junctions are found in animal cells and allow the passage of chemical and electrical signals as well as small molecules. Plasmodesmata are found in plant cells and allow proteins, sugars and ions to pass through.
Paracrine signaling affects nearby cells. A substance is secreted and is taken in and broken down by surrounding cells. This type of signaling can cause differentiation during development.
Signal transduction is the process of turning extracellular signals into a response. This begins when a signaling molecule (1st messenger) or ligand binds to a receptor protein on a target cell. The receptor protein changes shape which begins the transduction.
There are two types of signaling molecules: hydrophilic ligands and hydrophobic. Hydrophilic are unable to pass through the cell membrane and, therefore, attach to membrane receptors on the outside of the cell. The membrane receptors extend from the outside to the inside of the cell. Hydrophobic ligands can pass though the cell membrane and bind to intracellular receptors. When a signaling molecule binds to a receptor a secondary messenger is formed which can cause a response or become another second messenger.
The cell may respond in a signaling cascade which is an increase in enzyme reactions. An example of this is the kinase or phosphorylation cascade. In this kinase (an enzyme) is phosphorylated (adds a phosphate) which triggers the next kinase to do the same and so on until a protein is activated which causes a response. This response can be stopped by a protein phosphatase; an enzyme that dephoshorylates (removes phosphate).
Source: Pack, P. (2013). Ap biology. (4th ed., pp. 73-81). Boston: Houghton Mifflin Harcourt. http://www.bio.miami.edu/~cmallery/150/memb/c11x11enzyme-cascade.jpg