Mash (Conversion) Efficiency - How much of the available sugar was actually converted from the starch during the mash. This says nothing of your ability to separate that sugar during lautering/sparging. It is difficult to measure this and can be basically ignored if you're sure you've gotten good conversion (starch test).
Mash + Lauter/Sparge Efficiency - Assuming you converted all the starch to sugar, this is how much of the available sugars you were able to collect during the lauter/sparge function. Using a separation and sparge method that rinses "best" will yield higher efficiency. This value is easily measured by noting how much wort you've collected pre-boil (volume) and measuring its specific gravity (using a hydrometer or refractometer). These numbers will be compared against the theoretical maximum gravity.. More later. Note for you beersmith users: this type of figure is also called "efficiency into the boiler".
Brew House Efficiency - This measurement/figure takes into account your entire process and is the most indicative of how much your wort "costs". The volume/gravity measurements are taken post boil, or most accurately in the fermenter itself. This number will be lower the previous efficiency measurement because it takes into account any wort you may have lost in your tubing, absorbed into hop sludge, or spilled between the MLT and Kettle or the Kettle and fermenter.