The ECT button (which stands for "Electronically Controlled Transmission") 
changes the shift points for the automatic transmission.  When engaged,
the transmission shifts later, allowing the engine to get to a higher
power level and accelerate faster.  Gas mileage is usually reduced in 
this mode.  Use it when you need to get up to speed faster; for example, 
to merge into fast moving traffic.  The normal position for this switch 
is OFF.

OD, or OverDrive, is a 4th gear on the transmission that will engage 
when you are going above about 35 mph that lets the engine operate 
at a lower speed (RPMs) when all it has to do is maintain a high speed 
when you are going over flat, even highway.  This improves fuel 
economy.  Technically, any transmission gear with a ratio of less 
than 1:1 is considered "OverDrive".  OD can normally be left ON 
except when going up a gradual hill;  the overdrive tends to not 
give the engine enough power to maintain a steady speed.  This may
cause the transmission to annoyingly "hunt" between 3rd and 4th gear.