How do you use analytics to understand user behavior?

I've been building for some time now and feel that I never had a grip on using quantitative data to understand how users are using my apps, why they churn, etc. Thought I would ask in this forum and see if anyone can guide me a bit. For those of you who work on web SaaS or mobile apps, what type of analytics do you use to decide what features to work on, what changes to make and what to focus on optimizing?