Klaviyo is a Massachusetts-based software company which, per their recent S-1:
…provide[s] businesses of all sizes with powerful technology that captures, stores, analyzes, and predictively uses their own data to drive measurable, high-value outcomes. Klaviyo enables businesses to drive revenue growth by making it easy to bring their first-party data together and use it to create and deliver highly personalized consumer experiences across digital channels.
Klaviyo sells multiple offerings to their heavily e-commerce and heavily Shopify customer base. Their bread-and-butter, however, has for years been their email platform, their SMS capabilities, and their push notification capabilities.