Evens are east/west. Multiples of 10 are major freeways. Generally ordered so that the lower numbers are further south and the higher numbers are further north. Examples: I-10 goes from LA to Jacksonville and I-90 goes from Seattle to Boston
Odds are north/south. Multiples of 5 are major freeways. Generally ordered so that the lower numbers are further west and the higher numbers are further east. Examples: I-5 goes from San Diego to Seattle and I-95 goes from Miami to Maine.
Three digit freeways are auxiliary freeways usually local to specific metropolitan areas. The second and third digits represent the major freeway that it is associated with. The first digit rougly describes the type of auxiliary freeway. If the first digit is even, then it is a loop (usually for bypassing the metro area). If the first digit is odd, it is a spur. Those aren't very strict... in some larger metro areas, they sometimes run out of first numbers. Examples: I-94 goes straight through Minneapolis and Saint Paul, I-494 loops south, I-694 loops north and I-394 is a spur to the west.
There are a few other quirks. Sometimes there are E/W or N/S splits. I-35 happens to go through two pairs of "Twin Cities": Minneapolis-StPaul and Dallas-FortWorth. So, in both metro areas, I-35 splits into I-35W and I-35E.