When you 'gift' miles you are buying them for someone else (at 2.5 cents per mile).

When you 'share' miles you are paying US a fee to share some of your miles with someone else (at 1 cent per mile).

In both cases, whatever you 'gift' or 'share' is doubled up to 100,000 total in each category, until Dec. 31, 2009.

In total, in the current promo, someone could 'gift' you 50K, which would be doubled to 100K; they could 'share' 50K with you; which would be doubled to 100K; and you could 'buy' 50K, which would be doubled to 100K. All total, you could end up with 300K via the current 100% bonus promotion.