Tithes is a tenth of everything you get (gross). Offerings is above and beyond that.

Personally, I give to local church I attend, and I would also recommend that. 10% there, and then anything above that to other ministries or things.

Finally, I always try to make sure my "investment" matters. A "tithe" or "offering" isn't to something like a political organization. It should be to directly benefit others with the purpose of sharing Christ, leading others to him, or making him know to others.

Look at it this way – if you give $100 to XYZ organization and it helps 100 people, but none of them heard the gospel/die without knowing, was that really a good investment?