This document presents a general framework for building classifiers and clustering models using hidden topics to deal with short and sparse text data. It analyzes hidden topics from a large universal dataset using LDA. These topics are then used to enrich both the training data and new short text data by combining them with the topic distributions. This helps reduce data sparseness and improves classification and clustering accuracy for short texts like web snippets. The framework is also applied to contextual advertising by matching web pages and ads based on their hidden topic similarity.