Web analytics measures website performance and marketing/advertising efforts using key performance indicators (KPIs) such as number of visitors, conversion rate, revenue, and subscriber growth. Common goals for marketing sites include e-commerce, lead generation, and advertising. Effective web analytics involves acquiring visitors, measuring interactions, analyzing results, and testing improvements (AMAT framework). Key KPIs for e-commerce sites include visitor volume, conversion rate, order value, and ROI. Marketer KPIs measure marketing performance metrics like visits and conversions by medium and campaign.