Types of software

The one-line distinction: system software serves the machine, application software serves the user.

A utility program is a small system program that maintains, analyses or supports the computer, rather than doing a user's productive work. Utilities keep the system healthy and usable.

1. Antivirus — scans files and memory for malicious software, quarantines or removes what it finds, and monitors the system continuously. Examples: Windows Defender, Avast.

2. Disk defragmenter — over time a file's blocks become scattered across a hard disk, so the heads must move repeatedly to read one file. The defragmenter rearranges blocks so each file is contiguous, reducing seek time. (It is not used on SSDs, which have no seek time and whose cells have a limited number of writes.)

3. File compression tool — reduces file size for storage or transfer, and combines many files into one archive. Examples: WinZip, 7-Zip, WinRAR.

4. Backup and restore — copies data to another medium on a schedule, so that it can be recovered after deletion, corruption or hardware failure.

Others: disk cleanup, device drivers, disk formatting tools, screen savers, file managers.

Which is better while learning — an interpreter. Three reasons:

1. The feedback loop is shorter. Type a statement, run it, see the result. There is no separate compile step between writing and seeing what happens.

2. Errors point straight at the mistake. Execution stops at the line that failed, with the values in place, so the cause is usually visible. A compiler's list of forty cascading errors from one missing brace is far harder for a beginner to read.

3. A partly working program still runs. The working part executes and produces output even though a later part is broken — which is encouraging, and useful for testing an idea in pieces.

Why production systems still favour compilation: execution speed, lower memory use, and the ability to distribute a program without its source. In practice many modern languages do both — Java compiles to bytecode which is then interpreted or just-in-time compiled, taking the portability of one and much of the speed of the other.

Recommendation for a small bookshop — packaged software.

1. Cost. Custom development would cost many times the shop's likely annual software budget. A packaged accounts program is a small one-off purchase.
2. The requirement is ordinary. Sales, purchases, stock, GST and a ledger are needs common to every small retailer, and packaged accounting software already handles them all.
3. Available now. The shop can be running this week rather than after months of development.
4. Reliability and support. Software used by thousands of shops has had its bugs found already, and help is available from the vendor, from books and from other users.
5. Legal changes are handled for you. When tax rules change, the vendor issues an update — a custom program would need paid modification each time.

When a custom system would be justified instead: if the organisation's processes are genuinely unusual, if the volume is large enough that small inefficiencies matter, or if the software is itself a competitive advantage. None of those applies to a small bookshop.

Proprietary (commercial) software. Sold or licensed for use, with the source code withheld. The licence forbids modification, reverse engineering and redistribution. You buy permission to use it, not the program itself. Windows, MS Office, Adobe Photoshop.

Freeware. Distributed free of cost, for unlimited time, but the source code is not provided and modification is not permitted. The author retains full copyright. Adobe Reader, Google Chrome, VLC's binaries as distributed.

Shareware. Distributed free on a trial basis — for a limited period, or with some features disabled, or with reminders to pay. Continued or full use requires payment. Source is not provided. WinRAR is the classic example.

Open source software. The source code is published, and the licence explicitly permits anyone to study it, modify it and redistribute it (usually on condition that the same freedoms are passed on). Linux, Firefox, LibreOffice, MySQL, Python.

Is all free software open source? No. This is the commonest confusion, and the two examples above settle it. Google Chrome costs nothing — it is freeware — but Google does not publish the whole of its source, so you cannot inspect it, modify it or build your own version. Free of charge says nothing about whether the source is available.

Is all open source software free of cost? No, not necessarily. Open source licences guarantee the freedom to obtain and modify the source, not that no one may charge. Red Hat Enterprise Linux is open source and is sold, with the customer paying for support, certification and updates rather than for the code. Many open-source projects also sell commercial licences or hosted versions alongside a free one.

The two independent questions. It helps to see this as a grid:

All four boxes are occupied, which is the proof that price and openness are separate matters.

Why the distinction is worded so carefully. In the phrase free software, "free" refers to freedom, not price — the freedoms to run, study, modify and share the program. The term FOSS (Free and Open Source Software) is used when both properties hold at once, precisely because "free" alone is ambiguous in English.