Most people assume a domain name ends in .com so if you buy a domain name with one of the other extensions (.net, .info, .org, etc.), you’ll have some extra work to get people to remember that your site has a different extension. Don’t automatically assume you should only buy domains with .com, though. Many sites have done quite well with other extensions. (Look at us!) 

^ Mockapetris, P.V. (November 1987). "Domain names - concepts and facilities" (HTML). IETF Documents. IETF. doi:10.17487/RFC1034. Retrieved 24 July 2017. Relative names are either taken relative to a well known origin, or to a list of domains used as a search list. Relative names appear mostly at the user interface, where their interpretation varies from implementation to implementation, and in master files, where they are relative to a single origin domain name. The most common interpretation uses the root "." as either the single origin or as one of the members of the search list, so a multi-label relative name is often one where the trailing dot has been omitted to save typing.
Even if the term isn’t trademarked, don’t buy domains that are just a variation of another domain name. This means avoiding plurals if the singular is taken ( vs., hyphenating a phrase (, or adding “my” or some other preposition ( Alternately, you might consider buying these variations yourself and set them up so that if someone types one of the variations, they are redirected to your main site.