Decks of cards with a front and a back, a review session that shows the front and asks how it went, and a scheduler (the same one Anki uses) that brings each card back just before you would forget it. Vocabulary, anatomy, case law, the periodic table, your own notes. Everything lives on this device, and with a folder connected, in it.
Space shows the answer and then marks Good. Keys 1 to 4 are Again, Hard, Good, Easy. Escape stops.
The scheduler is SM-2. The algorithm behind Anki and SuperMemo, written out in twenty lines here. A card starts with an ease of 2.5; Good takes it to 1 day, then 6, then each interval times the ease; Hard and Easy nudge the ease down or up; Again resets it and repeats it in the same session. Twenty new cards a day, so a fresh deck of five hundred does not bury you.
Everything is on this device. In this browser's storage, and with a folder connected, as a file in Workspace/Cards. Export a deck as JSON to move it, with its schedule, to another browser.
It reads what Anki and spreadsheets write. One card per line, front, a tab, back. Anki's plain-text export, a two-column spreadsheet saved as tab-separated text, or a list you typed with " :: " between the sides all import.
No images or audio on cards. Text on both sides. A picture-based deck needs a different tool; for words, definitions, dates, formulas and facts, text is what you want to be recalling anyway.
Nothing is uploaded. There is no account and no sync. Open the network tab in your browser's developer tools and review a hundred cards; nothing goes out.
Whatever is due, which the scheduler keeps small: a card you know well comes back in weeks or months. New cards are the cost; twenty a day is the default and a good ceiling, because every new card is a review you owe later.
How it went. Good is the normal answer: you recalled it. Hard: you got there slowly, so the interval grows less. Easy: instant, so it grows more. Again: you did not, so the card starts over and comes back in this session.
Export it as JSON, which carries the schedule, and import it there. The tab-separated export carries only the cards, for a spreadsheet or for Anki.
In this browser on this device, and in your folder as a file if you have connected one. Clearing this site's data removes it; the export is the backup.