Tags are pieces of information stored in a stream that are not the content itself, but they rather describe the content. Most media container formats support tagging in one way or another. Ogg uses VorbisComment for this, MP3 uses ID3, AVI and WAV use RIFF's INFO list chunk, etc. GStreamer provides a general way for elements to read tags from the stream and expose this to the user. The tags (at least the metadata) will be part of the stream inside the pipeline. The consequence of this is that transcoding of files from one format to another will automatically preserve tags, as long as the input and output format elements both support tagging.
Tags are separated in two categories in GStreamer, even though applications won't notice anything of this. The first are called metadata, the second are called streaminfo. Metadata are tags that describe the non-technical parts of stream content. They can be changed without needing to re-encode the stream completely. Examples are "author", "title" or "album". The container format might still need to be re-written for the tags to fit in, though. Streaminfo, on the other hand, are tags that describe the stream contents technically. To change them, the stream needs to be re-encoded. Examples are "codec" or "bitrate". Note that some container formats (like ID3) store various streaminfo tags as metadata in the file container, which means that they can be changed so that they don't match the content in the file any more. Still, they are called metadata because technically, they can be changed without re-encoding the whole stream, even though that makes them invalid. Files with such metadata tags will have the same tag twice: once as metadata, once as streaminfo.
There is no special name for tag reading elements in GStreamer. There are specialised elements (e.g. id3demux) that do nothing besides tag reading, but any GStreamer element may extract tags while processing data, and most decoders, demuxers and parsers do.
A tag writer is called
An element supporting both can be used in a tag editor for quick tag
changing (note: in-place tag editing is still poorly supported at the time
of writing and usually requires tag extraction/stripping and remuxing of
the stream with new tags).