An ad group is a section inside your campaign where you group related keywords and ads together. Running one broad campaign with all your keywords mixed together makes it hard to know which keywords are performing well. When you split into smaller ad groups, each group targets a specific theme, so your ads are more relevant to what the person searched for. More relevance usually means a better quality score, which is Google's way of rating how useful your ad is, and that can lower your cost per click. Do you prefer a tight structure or do you keep things broad?