what is an api, if a make a db and a social site in asp.net and c#, sql host it. how can i ask others to make applications for it? i mean how is that possible, i have the code, so API, is it a software?

i want to make a site like facebook social networking site, but its proving to be very very difficult, Ajax java script and all that

please help by providing examples, codes, readable, guidance,

thank you, i has asked a similar question but deleted it as it was not clear

thank you