Others will provide a detailed breakdown soon, but for now, here is a highly simplified explanation:

CC BY means "Use it for free. modify it even. but give me (the artist/author) credit."

CC BY SA means "Use it for free. Modify it even. Give me  (the artist/author) credit. Furthermore, please share the modifications you make under these same terms."

ah... finally. thank you for your explanation. its really helpful

Here are some additional details for that overly simplified explanation:

Read up on both http://creativecommons.org/licenses/by/3.0/ and http://creativecommons.org/licenses/by-sa/3.0/

There are some nuances to these licenses that are discussed a lot. Namely, the "No Additional Restrictions" clause. It basically says the person using the asset cannot encrypt or hide or otherwise block the asset from being freely copied out of one project and used in another per the terms of the license.

It is often argued this clause means the asset cannot be used in games that are put on platforms like the apple app store since they may impose a form of DRM that prevents people from opening up the package and extracting the assets easily.

The debate is ongoing with no sign of ever arriving at a solid conclusion. If you're unsure, the best practice is to ask the artist who licensed the asset if they mind you using their work in such a fashion. Several artists have given a blanket waiver to this clause and don't need to be asked: http://opengameart.org/forumtopic/anti-drm-waiver

Alternatively, assets may be licensed under OGA-BY, which is essentially the same as CC-BY but without the No Additional Restrictions clause.

Another aspect of these licenses that is often discussed is whether or not the "Share Alike" clause of CC-BY-SA licenses requires a developer to share their game's entire source code or just the artwork derived directly from the asset. Again, the best practice is to ask the artist if you're unsure.