boarder / border are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
boarder: (noun) someone who forces their way aboard ship. (noun) a pupil who lives at school during term time.
border: (noun) a strip forming the outer edge of something. (verb) provide with a border or edge. (verb) lie adjacent to another or share a boundary.
boarder: b·AO·r·d·er
border: b·AO·r·d·er
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)