“The Extended Cut is an expansion of the original endings to Mass Effect 3,” BioWare said. “It does not fundamentally change the endings, but rather it expands on the meaning of the original endings, and reveals greater detail on the impact of player decisions.”
The new ending will add extra epilouge scenes and will flesh out the ending, hopefully giving players more clarification on the ending of Mass Effect 3. “It provides more of the answers and closure that players have been asking for. It gives a sense of what the future holds as a result of the decisions made throughout the series. And it shows greater detail in the successes or failures based on how players achieved their endings.”
Bioware also published this FAQ which can hopefully answer any questions that you might have.