Accept and Content-type are both headers sent from a client(browser say) to a service.
Accept header is a way for a client to specify the media type of the response content it is expecting and Content-type is a way to specify the media type of request being sent from the client to the server.
To expand on this:Accept header to quote the HTTP/1.1 RFC:
The Accept request-header field can be used to specify certain media types which are acceptable for the response.An example of an Accept header for a json request to a REST based service will be the following:
This is saying the response expected is a json content.
Content-Type to quote from the HTTP/1.1 RFC:
As a sample if a json is being sent from a browser to a server, then the content type header would look like this:
The Content-Type entity-header field indicates the media type of the entity-body sent to the recipient or, in the case of the HEAD method, the media type that would have been sent had the request been a GET.