2 Answers

From WIKIPEDIA

There are three forms of soldering, each requiring progressively higher temperatures and producing an increasingly stronger joint strength:
Soft soldering, which originally used a tin-lead alloy as the filler metal,
silver soldering, which uses an alloy containing silver,
brazing which uses a brass alloy for the filler.
The alloy of the filler metal for each type of soldering can be adjusted to modify the melting temperature of the filler. Soldering differs from gluing significantly in that the filler metals alloy with the workpiece at the junction to form a gas- and liquid-tight bond.[5]
Soft soldering is characterized by having a melting point of the filler metal below approximately 400 °C (752 °F),[8] whereas silver soldering and brazing use higher temperatures, typically requiring a flame or carbon arc torch to achieve the melting of the filler. Soft solder filler metals are typically alloys (often containing lead) that have liquidus temperatures below 350°C.
In this soldering process, heat is applied to the parts to be joined, causing the solder to melt and to bond to the workpieces in an alloying process called wetting. In stranded wire, the solder is drawn up into the wire by capillary action in a process called 'wicking'. Capillary action also takes place when the workpieces are very close together or touching. The joint's tensile strength is dependent on the filler metal used. Soldering produces electrically-conductive, water- and gas-tight joints.
Each type of solder offers advantages and disadvantages. Soft solder is so called because of the soft lead that is its primary ingredient. Soft soldering uses the lowest temperatures but does not make a strong joint and is unsuitable for mechanical load-bearing applications. It is also unsuitable for high-temperature applications as it softens and melts. Silver soldering, as used by jewelers, machinists and in some plumbing applications, requires the use of a torch or other high-temperature source, and is much stronger than soft soldering. Brazing provides the strongest joint but also requires the hottest temperatures to melt the filler metal, requiring a torch or other high temperature source and darkened goggles to protect the eyes from the bright light produced by the white-hot work. It is often used to repair cast-iron objects, wrought-iron furniture, etc.

From YAHOO ANSWERS. COM

Hard and soft are used, The term hard solder is generally reserved for those solders containing silver, Hard solder requires more heat than soft solder hard soldering can be done with standard acetylene soldering equipment if the materials being soldered are light, for joints of 1 1/4 inches and larger an oxyacetylene torch is required.
Soft soldering is the more common operation used by plumbers to join fittings and tubing together. solder may be purchased in many compositions, those concerning the plumber, usually contain the element lead,
some common soft solder types follow

50-50 this is 50% led and 50% tin
60-40 this is 60%led and 40% tin
95-5 this is 95% tin and 5% antimony

Yes, usually by the color as lead will oxidize faster