A handful of technical terms come up repeatedly across guides on compressing, scanning and converting files, and it helps to have them all defined clearly in one place rather than picking up a partial understanding of each one separately, guide by guide. Here they are, in plain language, with a pointer to where each one matters most.
DPI (dots per inch)
A measure of how much detail a scan or print captures per inch of the physical page. Higher DPI means more detail and a larger file; lower DPI means less detail and a smaller file. Most everyday scanning for on-screen reading needs far less DPI than people assume — covered in detail in our guide to the best DPI for scanning documents.
EXIF data
Hidden metadata a camera or phone attaches to a photo — location, device model, timestamp. Invisible in the image itself but readable by anyone who checks the file's properties. Worth stripping before sharing a photo publicly; see removing location and camera data from a photo.
Lossless vs lossy
Lossless compression (PNG) can be perfectly reversed with nothing discarded. Lossy compression (JPG) permanently discards some data to achieve a smaller file, ideally data you will not notice is missing. Neither is universally “better” — they suit different content, covered fully in what does “lossless” actually mean.
Vector vs raster
A raster image (JPG, PNG, WebP) is a fixed grid of pixels — it looks sharp at its native size and blurry if enlarged well beyond it. A vector image (SVG) is described mathematically as shapes, so it scales to any size with no loss of sharpness. Logos and icons benefit most from staying vector; photographs are always raster, since a photo cannot meaningfully be described as simple mathematical shapes.
Transparency (alpha channel)
The ability for part of an image to be see-through rather than a solid colour. PNG and WebP support this; JPG does not, which is why a transparent cutout saved as JPG unexpectedly gains a solid white background — see why a background-removed image must stay PNG.
Compression level (light, medium, strong)
How aggressively a tool reduces file size at the cost of some visual quality. These labels are not standardised across different tools — one tool's “medium” is not necessarily the same actual strength as another's, as covered in why different tools give different results.
Resolution vs file size
Resolution is how many pixels an image contains (its dimensions); file size is how many bytes it takes up on disk. A high-resolution image is not automatically a large file, and a low-resolution image is not automatically small — content and compression both matter independently of pixel count.
Encryption
Scrambling a file's content so it cannot be read without a specific key (usually a password). A properly password-protected PDF is genuinely encrypted, not just gated behind a login-style prompt — see password protection vs encryption for a PDF.
OCR (optical character recognition)
The process of recognising text within a scanned image so it becomes selectable, searchable text rather than just a picture of words. This is what happens behind the scenes when a scanned PDF is converted to an editable Word document.
Metadata
Information about a file that is not part of its visible content — EXIF data on a photo is one specific kind, but the term covers a broader category, including things like a PDF's author field, creation date, or software used to produce it. Most metadata is harmless, but it is worth knowing it exists separately from what you can see when you open the file.
Aspect ratio
The proportional relationship between an image's width and height, commonly written as a ratio like 16:9 or 1:1. Changing an image's aspect ratio without cropping first means either stretching it out of proportion or adding padding; matching the ratio through a proper crop keeps the subject looking correctly proportioned.
Bit depth and colour palette
How many distinct colours or shades an image format can represent. GIF's limited palette (256 colours) is why it shows visible banding on photographic content, covered in our guide on converting a photo to GIF; formats like JPG, PNG and WebP support millions of colours, which is why they handle photographic gradients far more smoothly.
Frequently asked questions
Do I need to understand all of these terms to use the tools on this site?
No, every tool is designed to be usable without any of this background knowledge; this glossary is here for when you want to understand what is actually happening, not as a prerequisite.
Which of these terms matters most for a typical everyday task?
For most people, understanding lossless vs lossy and roughly what DPI controls covers the majority of everyday decisions about images and scans.
Is a higher number always better for any of these (DPI, resolution, compression level)?
No, in every case there is a genuine trade-off with file size, and the right choice depends on how the file will actually be used, not on maximising the number.
What is the difference between resolution and DPI specifically?
Resolution is the total pixel count of an image; DPI relates pixel count to a physical printed size. The same pixel dimensions can represent different DPI depending on what physical size you are printing at.
Does this glossary cover everything I might encounter?
It covers the terms that come up most often across this blog's guides; if you run into an unfamiliar term in a specific guide, that guide typically explains it in context as well.
Is there a simpler way to think about all of this without memorising definitions?
Yes — for most tasks, focus on the practical question (does this need to stay perfectly sharp, or is some quality loss fine for a smaller file) rather than the underlying technical terms.
Are the tools that use these concepts free to use?
Yes. No sign-up, no watermark, and files are removed from the server automatically about an hour after processing.