1920 x 1080 is indeed a higher resolution than 1440 x 900. 1440 x 900 is not 1440p though. 1440p is 2560 x 1440 and a higher resolution than 1920 x 1080.
1440 x 900 is usually found on 17-19 inch LCD monitors.
1920 x 1080 is usually on larger screens. Even large HDTVs are 1920 x 1080.
2560 x 1440 is newer and more expensive. It's usually found on 27 inch and larger monitors and very high end TVs. There is very little content at this resolution and gaming at 1440p means at least a a single highest end graphics card and 2 in Sli or Crossfire is required to think about maxing games that high.