The Core bridge handles two channels (a "phone call") and bridge them together.
The conference bridge (meetme) handles several channels simultaneously with the support of an external timer (DAHDI timer). This is used not only by the Conference application (meetme) but also by the page application and the SLA system introduced in 1.4. The conference bridge does not handle video.
When two channels of the same type connect, the channel driver or the media subsystem used by the channel driver (i.e. RTP) can create a native bridge without sending media through the core.
Native briding can be disabled by a number of reasons, like DTMF being needed by the core or codecs being incompatible so a transcoding module is needed.