Android:リソースファイルの解像度(dpi)

ここに記載されています。

ldpi: Low-density screens; approximately 120dpi.
mdpi: Medium-density (on traditional HVGA) screens; approximately 160dpi.
hdpi: High-density screens; approximately 240dpi.
xhdpi: Extra high-density screens; approximately 320dpi. Added in API Level 8
nodpi: This can be used for bitmap resources that you do not want to be scaled to match the device density.