Good question, which has no short answer.
Hardware companies have a numbering/naming strategy that supposedly conveys information about the features and capability of the product. This normally changes every few years when they run out of numbers and then have to start afresh.
Anyway, the current Nvidia scheme uses 3 numbers. The first number identifies the product family, while the last two identify where that product fits inside the family.
Using your example, the 610 and the 635 are both part of the 6 series (or 600 series, depending on how you look at it). This means they have more or less the same features and use the same architecture. Comparing the other numbers, since 35 is higher than 10, the 635 should be faster than the 610.
Hope this helps a bit.