The current incarnation ask.com is focused on searching for web pages. The previous incarnation of ask.com - AskJeeves - was known for its ability to work with natural language queries. The rest of a natural language query with AskJeeves was a list of relevant web pages.
With Yedda, you can ask your questions in natural language, but that's where the similarity to AskJeeves starts and ends. Yedda's main mission is to highlight your question to the people who may have the answers to it, and to deliver their answers to you.
Ask.com is a great search engine, go ahead and use it when you're looking for web pages. But if you're looking for an answer - personal knowledge, from real people, that fully understand your question and the responses of their peers - you're in the right place :)
While we're at it, when you ask a question, we will of course share with you the list of similar questions which were already answered through Yedda in the past. To do that, we use a semantic search, which is somewhat similar, in function if not in implementation, to the AskJeeves functionality.
eHow is a great how-to manual for quite a lot of topics. The content on eHow it staff-edited, and is quite extensive. If you're looking for a generic how-to information on a common topic, ehow is a pretty good resource for that. However, if you're looking for a how-to solution for your own specific issue with your own specific set of circumstances, Yedda might be a better place for you.
Of course, Yedda covers much more then how-to information (for example, "Is Visual Studio Tools for Applications replacing VBA" does not really fall under the how-do category :))