What is R-O-H-S? ROHS stands for reduction of hazards substances and in case of soldering, that mainly means lead. Lead is a poisonous heavy metal which is toxic to humans, mostly due to ingestion. If you eat a lot of lead, you may get lead poisoning, and this is common in young children. So, if you have young children present in your household, you will want to take some precautions with the solder that you use.
Also, there has been a political push to make sure that less lead is getting into the environment and in order to do so, this ROHS program has been implemented all over Europe so that most the electronics sold there no longer contain lead. And so, it is necessary to use an alternative formulation, such as lead-free solder.
A lead-free solder comes in different formulations and I have here with me 1 of the most common formulations, which is mostly tin. This is a 99% tin solder that has a melting point that is slightly higher than tin-lead solder, so it is a little bit more difficult to work with, and it can be a little bit of a learning curve in order to begin using. But it can be used with standard soldering equipment, and it may be worthwhile to implement in your soldering project.
This is lead-free solder. This is a common formulation with 99.2 % tin and it is alloyed with 0.3% silver and 0.5% copper to improve its mechanical properties. This is a very common formulation that you may find at RadioShack, and it is very easy to begin using if you are moving from a lead solder.
You may find that it is easier to learn with the lead solder. However, if you have concerns about health; if you have young children in your household; you may simply want to move to the lead-free solder and learn how to work with that particular material. The melting point's a little bit higher, so you may need to work with hotter soldering iron, you may need to increase the heating times, but for the most part you should get the same results as you would with leaded solder.