The active sequence on the timeline, usually just SEQUENCE 1, is what export uses to produce a video file, DVD, YouTube upload, etc. There is no "export sequence" dialog.
However, if multiple sequences have been created, they can be exported en masse with FILE|EXPORT ALL SEQUENCES. Each sequence will prompt for format/file changes.
See this thread.
A sequence is an arrangement of clips on audio and video tracks. Each project can have multiple sequences. Sequences can be viewed in either Timeline or Storyboard mode.
A sequence can be used as a clip inside another sequence. For example, each scene could be edited in its own sequence, then all the sequence clips could be ordered in another sequence to assemble the final video. Note that subtitles added to a sequence will not show when that sequence is used as a clip in another sequence.