When used correctly in regards to the entertainment-industry casting , "call" and "audition" do usually mean the same thing. Common phrases: "Open call," "Open Audition," "Audition call," etc.
Info about these types of opportunities (open call auditions) is usually found in the Casting/Job articles and listings in entertainment "trade" magazines/websites like Back Stage (http://BackStage.com), Show Business Weekly, Actor's Access, LA Casting, Casting Workbook, and even on job sites like Craigslist.com and in the "local interest" and entertainment sections of regional newspapers, etc.
Keywords to look for on these sites include "Casting Notices," "Casting Calls," "Production Listings," "Breakdowns," "Auditions Calendar," "Casting Search," etc.
Typically an "open" call/audition is one that doesn't require an appointment and/or doesn't require membership in one of the unions and/or doesn't require representation by an agent.
Alternately, an "Audition by appointment" or "Audition by appt." means that the call isn't "open" -- so you or your agent will either need to make an appointment/time-slot reservation before going to the audition or you'll have to submit your photos and resumes via mail or email and then wait and hope that the casting director calls you back with an audition invitation/time-slot.
A "submission call" is a casting notice/breakdown that's calling for/requesting submissions (by mail or email or drop-off). For this kind of call, if the casting director/producers find your submission to be of interest, then they'll contact you later for an interview and/or audition.
And a "union call" is an audition that is primarily intended for union actors. Typically this is spelled out as "SAG auditions," "Equity principal auditions" (EPA), "Equity Chorus call,"Equity performer auditions," "AFTRA auditions," etc. In these cases the word "call" is often used interchangeably with the word "auditions."
And a "call back" is a follow-up audition: The actors that do the best at the first round of auditions are usually invited to a "call back" audition, and sometimes a "second call back" and "third call back," etc., until the producers finally decide on exactly which actor they want for the role.