The conjunction occurs when two or more planets are on the same degree on the ecliptic, that is, when planets are in the same place in the sky from the vantage point of the earth. As the name implies, the energies of the planets involved form a hybrid, fuse together as one.
However, with aspects there is what is called an orb of influence, which is, the degree of give and take that is allowed for the aspect to be still considered an aspect. The number of degree can differ from astrologer to astrologer and it usually comes down to personal experience and preference. For example, a true conjunction will have two or in some cases more planets (or the angular houses) sitting in exactly the same degree of a sign. However, most astrologers allow a little bit of give and take between aspects and it could be that your planets are sitting 2 degrees apart and are still considered conjunct. Obviously the closer the degree of orb the more impact the conjunction will have.
Naturally because there are 30 degrees to the astrological signs most conjunctions are in the same sign as can be seen in this example here. However, it is possible to have conjunctions in different signs if one planet is at the very end of one sign and the other is at the very beginning of the next sign.
Planets can also make aspects to the different angular houses. The angular houses are the Ascendant, Descendant, Medium Coeli and Immum Coeli. The Ascendant is on the cusp (line between houses) of the first house and is directly opposite the Descendant, the cusp of the 7th house. The Medium Coeli is on the cusp of the 10th house and opposite the Immum Coeli, which is on the cusp of the 4th house.
Conjunctions usually intensify what qualities the planets involved bring into play in a chart. This can be good or bad depending on the planets involved and in what houses they are in.
What is often neglected is that the planets forming a conjunction act reciprocally, that is, their energies act synergistically, each planet’s energy acts upon the other. It is easy to fall into the trap of thinking that one planet (often the outer of two planets) acts upon the other (usually the inner of two planets), however, the energy exchange is much more dynamic than this. The conjunction is the most powerful aspect simply in terms of the amount of energy involved.
Because it doesn’t necessitate conflict with the outer world, the conjunction can lead to self-absorption, withdrawal, a preoccupation with self-interest, or an assertion of the inner will out into the world at large. There is a latitude of freedom in the conjunction not found in the square or opposition–the owner has liberty to deal with the energy of the conjunction as he or she sees fit–however, the shear potency of the energy forces the individual to deal with this aspect in a significant way.