Join 

Menu: Geometry->Edit→Join

With this option you try to join connected entities.

Join lines:
To join the lines of a selection that are connected with 'similar tangent' (with an user tolerance).
Note: the lines are not joined if the connection point belongs to other lines, has some condition applied, or the lines have set an structured number of divisions.

Join surfaces:

• Rebuild by boundary:

A group or connected surfaces must be selected in order to try to create a single approximated surface.

• • QuickJoin (default option)
  • NoQuickJoin. More parameters are available.
    • IsProjectable : Yes or Not
    • WeightBoundary: Yes or Not
    • NumApproxPoints
    • Tolerance
• Join only coplanars:

A group of surfaces must be selected, and automatically are detected subgroups of planar parts that are joined in single planar surfaces.

• • TolAngle could be set to determine when two surfaces are coplanar

Join volumes:

Two or more volumes connected by shared surfaces are joined in a single one.

By default, if 'NoDeleteOriginalVolumes' was set, the source volumes and its dependent entities will be deleted.