Your library owns things: books, magazines, puzzles, cake pans, snow shoes, movies, recorded music, etc. In some cases you probably own multiples of exactly the same thing (several copies of Strega Nona, for example). Each of the different things you own is represented in your Koha system by a bibliographic record (generally referred to as a bib record). Every individual copy of each thing you own is represented by an item record which is attached to the bib record it is one of. For example, you may have 3 copies of Peyton Place (published in 1956 by Messner, 372 pages long). You will have 1 bib record in your system for this (oclc record #00289487) which includes information about the thing – who wrote it, how many pages it has, where and when it was published, subject headings that describe what it is about, etc.
Attached to this example bib record there will be 3 item records (because you have 3 of the exact same book). Each item record will include a call number (they might or might not all have the same call number, depending on where you keep the different copies), and a barcode that uniquely identifies each physical item. The item record might also have notes that describe something about the specific copy—that it is water damaged, or that it was signed by the author, or that it was donated to your library by someone, etc.
The bibliographic record will have an ITEM TYPE assigned to it (and included as part of the bib record) that tells you the kind of thing it is: a book, a movie, a museum pass, etc. These are broad categories of things – Video games, for example. More detailed information (it is a wii game, or an Xbox game, for example) is included in the bibliographic record and is not given a separate ITEM TYPE. You can think of this like the chapters in a cookbook. In The Joy of Cooking you will find recipes (think of them as bib records) for blueberry pie, pumpkin pie, pecan pie, etc. These recipes will be in a chapter (think of it like an item type) called Pies. This same ITEM TYPE will be included in all the items that are attached to the bib record. There is a defined list of ITEM TYPES in the nhaisLOCAL system that is used by all the different nhaisLOCAL catalogs.
Groups of ITEMS may be organized conceptually (as opposed to physically) into defined COLLECTIONS. These might include adult fiction, juvenile non-fiction, reference, local history, music, etc. Each individual library can have a defined list of COLLECTIONS in the nhaisLOCAL system that may be used to group items. The item record will include information about what COLLECTION the item is part of. This is optional, an item need not be part of a COLLECTION, but using COLLECTIONS will allow you to create reports about these groups of stuff and may help patrons find similar materials.
Groups of ITEMS will be organized physically in your library. Each nhaisLOCAL library may define a list of LOCATIONS that they can then use to explain where in the building items can be found. This list includes things like Stacks, Reference Room, In Processing, Children’s Room, Librarian’s Office, Bat Cave, Teen Area, etc. This is optional, an item need not have a LOCATION attached to it, but using LOCATIONS can help both staff and patrons find things. The item record includes a field that indicates what LOCATION the item belongs in. Frequently this information is also, in an abbreviated form, part of the call number for the item.
Your library has all this stuff so that people can use it. Those people are called patrons in the nhaisLOCAL systems. There are different groups of patrons (children, adults, out-of-town borrowers) who can do different things (renew materials, borrow DVDs, etc.). What specific things an individual person can or cannot do in your library is defined by the PATRON TYPE they are assigned. Each nhaisLOCAL library defines their own list of PATRON TYPES which they can use to group their patrons. The privileges your patrons have are defined in the configuration of each PATRON TYPE and may vary for different ITEM TYPES. For example, you can specify that a patron with the PATRON TYPE “adult” can borrow items of the ITEM TYPE “movies” for 2 weeks, with no renewals allowed. Or 1 week with 8 renewals, or whatever you want. The rules for the ITEM TYPE ‘in-library equipment” can be totally different.