Conjunctions are about unity. The energies combine (i.e., add up) and move forward together in the same direction, to achieve the same objective. It's about working side-by-side. Like trines, conjunctions establish links and help create bonds. They bring people together.
Conjunctions also remove boundaries, which can be stressful for independent or introvert minds. You don't always want to join groups or be forced to. They can blur situations and sometime bring confusion. The limits are not clear anymore. Some energies don't blend well together too, thus conjunctions may create or increase an existing stress between them.
The interpretation of conjunctions in birth charts, transits and synastry is more or less the same, with the exception that in synastry, when Jupiter, Uranus or Lilith are involved, this may be push things beyond acceptable limits or create boundless situations.
Other major aspects: opposition, sextile, square and trine