Difference between Short Description and Long Description

A short description (up to 70 characters) is the brief summary of the products or services that you want to advertise. As you have the character limit of 70 you should be so creative in expressing your service.

Short descriptions are now required when you create or modify an ad.
A long description (up to 190 characters) is a detailed summary of the products or services that you want to advertise. It will not be displayed on the Yahoo! search. It is displayed on partner sites that can accommodate longer descriptions.
Long descriptions are optional

How should I organize my keywords in ad groups?

These are best practices
• Make sure all keywords in an ad group are very closely related to one another. The “tighter” the ad group is, the more likely you are to write ads that relate to all of your keywords.
• Don’t put the same keywords in multiple ad groups, because they will compete against themselves. Two exceptions to this practice are using the same keywords in both geo-targeted and non-geo-targeted campaigns, or using the same keyword for different seasonal campaigns. For example, you can use the keyword “gift baskets” for both a Valentine’s Day campaign and a Mother’s Day campaign.
• Keep what’s working! Remove, or just move, low performers so they don’t negatively impact your ad quality and you don’t waste too much time monitoring them.