この is classified as a ‘pre-noun adjectival’ in Japanese. These are a special type of word that do not conjugate. This means that they will always appear in the same form. この comes from the same family of words as これ and ここ, and is used to identify a ‘thing’ that is near the speaker.

• このスーツはあなたのですか？
  Is this suit yours?
• この仕事（しごと）は大変（たいへん）だ。
  This work is difficult.

As the name ‘pre-noun adjectival’ suggests, these words will always appear before a noun, and describe that noun in some way.