Web analytics involves collecting and analyzing data generated by visitors' web browsers while they are viewing webpages. The resulting data provides insight into how visitors use the site.
Google Analytics is a widely used free analytics tool for website traffic.
MixPanel's analytics platform focuses on mobile and sales funnel metrics. A developer builds what data points need to be collected into the server side or client side code. MixPanel captures that data and provides metrics and visualizations based on the data.
KISSmetrics' analytics provides context for who is visiting a website and what actions they are taking while on the site.
CrazyEgg is tool for understanding a user's focus while using a website based on heatmaps generated from mouse movements.