Question: If I use diodes in a switched DC-DC converter like the output diode on a Buck converter, and the converter switching rate is 50KHZ, how do i choose the right diode? what do I have to look for in the diode DataSheet?

Answer: 
Current capability (which will depend on temperature), reverse recovery time, forward voltage at the expected currents, reverse leakage (heavily dependent on temperature). PIV (Peak Inverse Voltage) rating. 

Generally you'd use a Schottky diode for 50kHz unless it was relatively high voltage, then you'd use an ultra-fast recovery silicon diode. 

The considerations are similar regardless of voltage or current, but the parts will be quite different for a 5V/1A circuit vs. a 500V/0.1A or 2V/100A circuit (in the latter case, you'd probably want to use a synchronous rectifier and eschew the diode entirely).
For step down converters, choose max V from 40 to 100V then max Iavg, package ... SMT preferred over THT, low voltage drop at peak I x Thermal Resistance (deg C/W) < 50'C and fast recovery time <500ns. This reduces options from thousands to hundreds. Then choose stock on hand and price. For 50kHz anything <500ns is good. Best depends on all the above tradeoffs, but don't ignore temperature effects.

[DG options](http://www.digikey.com/product-search/download.csv?FV=fff40015,fff8007f,fffc001f,fffc0059,fffc040a,e480002,e480006,e480014,e480016,e48001c,e480020,e480023,e48002f&k=Diode&mnonly=0&newproducts=0&ColumnSort=0&page=2&stock=1&pbfree=0&rohs=0&quantity=0&ptm=0&fid=0&pageSize=25)