The Gallup Daily Tracking Poll - the first one in the election cycle that pits Obama versus Romney - is based on a rolling, five day average. It shows Mitt Romney up on President Obama by two points.
The CNN poll, on the other hand, is a snapshot poll taken over a three day period with weighted averages. This survey gives Obama a 52-43 advantage to the president.
What gives? Part of the reason may be found in who the surveys question. Gallup asks registered voters only while CNN questioned 910 registered voters out of 1015 respondents. There is also a question of how both polls choose respondents. Gallup asked 2265 random registered voters, with at least 400 cell phone users included while CNN used only 256 cell users. (The inclusion of cell users reflects the fact that more Americans are ditching their landlines.)
Perhaps the biggest difference is in the nature of both polls. A rolling average is far less volatile over the long run than snapshot polls taken in 2-3 day cycles. It doesn't make Gallup any more "accurate" but it probably gives a better account of the broader peaks and valleys of a candidate's popularity.
Follow RealClearPolitics.com for both the Gallup and Rasmussen daily tracking surveys.