Anions are atoms or radicals which are a group of atoms, that have gained electrons. Since they now have more electrons than protons, anions have a negative charge. For example Chloride ions Cl- , Bromide Br- , Iodide I-. These are monovalent anions as it has a combining capacity with only one ion of Hydrogen. Similarly there are bivalent anions, etc.
Anions are termed so because they get attracted towards the Anode(the positive electrode).
All Anions tend to accept a proton H+ thus they are categorized as bases