Press-ons are pre-formed full nails applied at home with adhesive tabs or nail glue. The modern generation deserves none of the drugstore condescension the name inherits: today's press-ons come in proper shapes and sizings, often hand-painted, and a well-fitted set photographs convincingly next to salon work. Tabs give a day or two of wear and painless removal; glue gives up to a week or so with more care needed at removal (soak, never pry).
Their honest role is commitment-free drama: an event weekend, a trial run of XL length before paying to live with it, a job that forbids long nails on weekdays. What they do not offer is the three-week durability of a cured salon system — different sport entirely. Kept and reused carefully, a good press-on set is also the cheapest recurring manicure there is.