Ans1  a) An information retrieval system is a software programme that stores and manages information on documents, often textual documents but possibly multimedia.The system assists users in finding the information they need.
b)  The task of sequential pattern mining is a data mining task specialized for analyzing sequential data, to discover sequential patterns. More precisely, it consists of discovering interesting subsequences in a set of sequences. 
c) A web search engine is a software system that is designed to search for information on the World Wide Web. The search results are generally presented in a line of results often referred to as search engine results pages.
d) Web Usage mining is the process of applying data mining techniques to the discovery of usage patterns from Web data, targeted towards various applications. 
The three phases for web usage mining are:
– Preprocessing,
– Pattern discovery, and
– Patterns analysis.
e) Clustering is the task of dividing the population or data points into a number of groups such that data points in the same groups are more similar to other data points in the same group than those in other groups. In simple words, the aim is to segregate groups with similar traits and assign them into clusters.
Ans2 a) A decision tree is a map of the possible outcomes of a series of related choices. It allows an individual or organization to weigh possible actions against one another based on their costs, probabilities, and benefits. A decision tree typically starts with a single node, which branches into possible outcomes. Each of those outcomes leads to additional nodes, which branch off into other possibilities. This gives it a treelike shape.
b) association rule mining it is a procedure which aims to observe frequently occurring patterns, correlations, or associations from datasets found in various kinds of databases such as relational databases, transactional databases, and other forms of repositories.
The main applications of association rule mining: 
Basket data analysis - is to analyze the association of purchased items in a single basket or single purchase as per the examples given above.
Cross marketing - is to work with other businesses that complement your own, not competitors. 
Catalog design - the selection of items in a business’ catalog are often designed to complement each other so that buying one item will lead to buying of another. So these items are often complements or very related.