PlugInPortnoy
The hammer represents production or productivity. Units, buildings and wonders require a certain amount of hammers in order to be created. If you are wanting to build a Warrior for instance; a warrior costs 15 hammers (i think), if your city produces 1 hammer per turn it will take 15 turns to build a warrior.
Each content or happy citizen of each of your cities can work a tile within the city's radius. From working a tile you can either gain growth (food), productivity (hammers) or commerce (coins). Instead of having your citizens work tiles you can remove them and make them into specialists. The basic specialist will only produce one hammer per turn. If you construct a cultural building such as a theatre you can make them into an artist (specialist). Per turn an artist produces; extra culture for your city, 3 great person points and I think some gold also. Accordingly artists are good if you want more culture, great person points or gold.
If your cities are beside the sea you can build a lighthouse which increases the output of each water tile by 1 food, assuming you assign a citizen to work them. A city needs to be right beside the sea in order to build a lighthouse.
You may also create work boats. Assuming you have a water resource in your cities radius such as fish or clam, the work boats will allow you to build fishing nets or something which will increase the food output of that tile.
A great merchant will add 1 extra food per turn to a city if you settle him in it.
Libraries boost a city's research by 50% or something. If a city is producing a good amount of research then a library will increase its research output significanty - which really is crucial if you want to keep up with your rivals in the technology race.
Courthouses save you money if your city has high maintenance costs. Cities that are far away from your capital generally have quite high maintenace, so building courthouses in these can be useful if you need to save money. The same goes for large cities. Large cities have quite high maintenance costs, courthouses are useful in these too.
The only way to get rid of the its too crowded complaint is to starve your city (resulting in lower population). This generally isn't a good idea. A better idea is to make those people happy again by building happiness improvements - colluseum, temples etc. More ways of making people happy are by getting happiness resources - gold, sugar etc. Having a state religion in your city will make people happy.
----
Your questions are quite basic. If you have access i'd recommend reading the manual. If not some people here have taken time to write helpful articles. Have a look at the FAQ on the civfanatics mainpage or some of the stickies on this forum.
mwr
That's quite a vague question. It will probably depend on which mod you downloaded. The mod may come with a 'readme' file or something, read it. If not have a look in the creation and customization forums for a thread related to the mod. There should be instructions there, if not you've got more chance of finding help there anyway (ask).
fu greeneyedzombie!!