HTTP cookies (also referred to as tracking cookies, or simply cookies) are small pieces of text, stored on a user's computer by a web browser; and they contain the user's settings, shopping cart contents, or other data used by websites. They are transmitted as parcels of text sent by a server to a Web client (usually a browser) and then sent back unchanged by client each time it accesses that server. HTTP cookies are used for authenticating, session tracking (state maintenance), and maintaining specific information about users, such as site preferences or the contents of their electronic shopping carts. The term "cookie" is derived from "magic cookie," a well-known concept in UNIX computing which inspired both the idea and the name of HTTP cookies. Some alternatives to cookies exist, but each has its own uses, advantages, and drawbacks. Cookies are also subject to a number of misconceptions, mostly based on the erroneous notion that they are computer programs that run on the browsing computer. In fact, cookies are simple pieces of data that affect the operation of a web server, not the client, and do so in very specific ways. In particular, they are neither spyware nor viruses, although cookies from certain sites are described as spyware by many anti-spyware products because they allow users to be tracked when they visit various sites. Most modern browsers allow users to decide whether to accept cookies, but rejection makes some websites unusable. For example, shopping carts implemented using cookies do not work if cookies are rejected. In computer science, a cache (pronounced /kæʃ /) is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive to fetch (owing to longer access time) or to compute, compared to the cost of reading the cache. In other words, a cache is a temporary storage area where frequently accessed data can be stored for rapid access. Once the data is stored in the cache, it can be used in the future by accessing the cached copy rather than re-fetching or recomputing the original data. A cache has proven to be extremely effective in many areas of computing because access patterns in typical computer applications have locality of reference. There are several kinds of locality, but this article primarily deals with data that are accessed close together in time (temporal locality). The data might or might not be located physically close to each other (spatial locality).
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