Why are there so many names for electronic components in eagle cad?
They are different, but the differences may not matter depending on what you're using them for. 

For example, the 1N4148 is rated for 100V peak reverse voltage, while the 1N5400 is rated for only 50V. Are you expecting high reverse voltages for your diode? If so, then the 1n4148 would be the better choice out of these two.

Conversely, the 1N4148 is rated for 150mA average rectified forward current, while the 1N5400 is rated for 3A. Are you expecting very high currents? If so, then the 1N5400 is better for that purpose.

There will be tradeoffs between all of the diodes, and part of the "engineering" is choosing the right one. If you only need a general purpose, medium voltage medium current diode, then the 1N4004 is a reasonable choice.