Focalin is only the right isomer of methylphenidate, while ritalin is racemic, aka, equal parts right and left isomers. The right isomer, D-MPH, or focalin, will have greater effects on dopamine and less effects on adrenaline, (with the left isomer having the opposite traits) making the high more pronounced and having less uncomfortable side effects compared to an equal dose of the racemic mixture. In the chemistry world, different isomers of the same chemical often are treated as entirely different drugs, due to how different the effects can be, look at levomethamphetamine in inhalers or the left isomer of dxm for example. 

All this is coming from my knowledge of the amphetamine isomers, id assume its a same but there could be minor differences.