Wide angle lenses are pretty self explanatory, they allow you to gain a wider field of view. Landscape photographers tend to use these lenses to make sure they get the full scene of what they are photographing. Keep in mind that the lower the number focal length, the more that will fit in your frame. Wide angle lenses tend to distort photos (especially of faces), which can fixed moderately post-processing. They are very versatile lenses and tend to be really affordable.
Types of Wide Angle Lenses: 14mm, 20mm, 21mm, 24mm, 28mm, 35mm