This document summarizes aggregation systems for big data. It discusses how aggregations turn raw data into condensed, human-understandable information through techniques like time series, top-k, cardinality, and quantiles. It also covers abstraction concepts like monoids that allow different data types to be aggregated. Finally, it outlines common components of online incremental aggregation systems and provides examples like Twitter's Summingbird and Google's Mesa that compute aggregations incrementally in scalable key-value stores.