Ironically I had a page long writeup for you but GG I refreshed the browser by mistake. This time around, I'm going to keep it short.
Back in the day, programs were large unwieldly things, Kline (Thousand Line) was the billing style. To keep track of what these large complex programs did, Flowcharts were born. This allowed you to trace the path of information from start to finish in a program. For projects it looked great and was a nice visual to keep everyone on track. It was also supremely helpful when troubleshooting problems because you could see all the points the data followed and track at which point it got screwed up.
Fast forward to 2008 and Flowcharts are largely dead from a programming perspective. Object Oriented systems use a variety of new types of Flowcharts, that while perform the same function in practice, allow for the use of the Object Oriented style of programming.
Schema is specific to database, but I have seen people shoehorn a Schema to cover a web page system and it wasn't pretty. Let's keep it simple. Schema is Database, and Database only. A schema is similar to a flowchart but for a database. All of your tables in your database will be prepresented in your Schema. Each table will have its name, and all the fields listed in it. Sometimes these fields will also have their datatype listed on the Schema, but not always. The Schema will also provide lines to indicate which fields are linked to each other. These links tell you that Field 4 from Table 6 is linked to Field 1 in Table 14, and what kind of link it is. This is important for Projects because before any code is written or even the Database itself built, a Schema must exist. Without a Schema most database projects would be too unwieldly to comprehend where everything connects. Instead your Engineers and Architects sit around and build the Schema, and the Schema serves as the blueprint when all the tables are being written, setup, built, and put into the database. It also tells you which fields to link and where to link them. It also tells you what need to come first because primaries need to be built before secondaries and foreign fields. If Table 1 has a Primary Key that is referenced in Table 6, 7, and 8. Then Table 1 needs to be built before Table 6, 7, or 8, otherwise they would be unable to reference the Table 1 Primary Key.
Schema's also allow the coding and creation tasks to be sent to another team that does nothing but write code and build tables. In some cases, there are modern tools that you feed the Schema into and it outputs the code to the most part, which cuts down on the time required and possibility of stupid syntax errors when creating the database system.
For a visual of a schema see http://en.wikipedia.org/wiki/File:Media ... schema.svg
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