All about IC50 and EC50

In layman terms IC₅₀ or EC₅₀ values are quantitative measures of a drug needed for a particular biological function. IC₅₀ or EC₅₀ values should be as small as possible;  i.e. the smaller the value, the better is the drug (compound).  This goes by the definition of these terms – IC is the inhibitory concentration and EC is the effective concentration.  If the amount (relates to concentration) of drug (compound) needed is less for the necessary biological response, then that drug (compound) should be good.

Conceptually IC₅₀ and EC₅₀ are not different. 
In general, EC₅₀ is used when dealing with cell-receptors. 
IC₅₀ is used when dealing with enzymes.

IC₅₀  [half maximal inhibitor concentration]      
IC50 is the drug concentration causing 50% inhibition of the desired activity.

EC₅₀ [half maximal effective concentration]
EC₅₀ is the drug concentration which induces a response halfway between baseline and maximum.

If you are dealing with compounds which are agonists for a particular cell-receptor, then binding of the agonist to the cell-receptor will activate a biological process which can be measured. 

In a competitive binding assay, say antagonist is bound to the cell-receptor deactivating a biological process (100% deactivation);   now adding different amounts of agonist this antagonist can be displaced causing activation and biological response of different degree (0% to 100%) based on the amount of compound added.  Concentration required for observing 50% biological response is EC₅₀.

Remember

In vitro biological activity data: IC₅₀ for inhibitors and EC₅₀ for agonists/antagonists.  Depending on the design of assay protocol for antagonists IC₅₀ will also be used sometimes.  Smaller the IC₅₀/EC₅₀ number - better is the compound.  The activity data is expressed in 'nM' (nanomolar) or 'mM' (micromolar) units.

You obtain IC₅₀ or EC₅₀ data only for vitro studies (both in enzymatic assays and cell-based assays).    Enzyme/receptor data vs cell-based assay results are differentiated by providing the data in this way -  IC₅₀ / EC₅₀ (enzymatic assay) and IC₅₀ / EC₅₀ (cell assay). 
[IC₅₀ or EC₅₀ values are no way related to in vivo data]                                                                               

IC₅₀ or EC₅₀ are just numbers.  The information these numbers give is that the compound is providing some kind of biological response by binding to the enzyme.  However, one should keep in mind that IC₅₀ or EC₅₀ values will not give you information about where and how the drugs (compounds) are interacting with enzyme/receptor (mode of interaction).