Online maps have evolved greatly over the years and are very powerful today with many features. There is also a lot of confusion about these maps, and it helps to understand a bit how they work.
Before making a map, one needs geographic information. Where are all the cities, rivers, houses, political boundaries, bicycle paths, etc. There are lots of sorts of data we may want, and all of this is called vector data. Large amounts of vector data are collected and owned mostly by governments and a small number of companies. This data is collected by on-the-ground surveying, using GPS, with aerial imagery, etc. Most governments handle the data for their area only; for a whole-world set of data there are now only two companies that own a decent set: Nokia (through buying Navteq) and TomTom (through buying Teleatlas).
Most owners of vector data charge very high prices for it, which is what lead to the creation of OpenStreetMap, which is best-described as being a ”Wikipedia for maps,„ a website where anyone can contribute and add to the map about the places that they know. OpenStreetMap (or OSM for short) is now a sort of third contender, and the only free one, and so there are an increasing number of maps and tools that use OSM data. OpenStreetMap is relatively new, and so almost any map you see from a major map website or on a GPS device or mobile phone is showing you maps made from vector data from Nokia or TomTom.
The popular map websites (e.g. Google Maps, Mapquest, etc.) first acquire vector data, usually by buying it. In many cases they get this data from multiple sources and have to do some work to merge them together. Nokia's vector data powers maps on Bing Maps, Yahoo, Mapquest, among others, and TomTom's vector data powers Google’s. Google has also begun collecting vector data themselves, and their own data is the primary source for their maps of the United States and Canada.
After merging their vector data, this new combined vector data is used along with complex algorithms and map styles to render maps; as in, create the map images that you see. Most map websites today take these map images and slice them into tons of small squares called ”tiles„ which allow them to load only the portions of the map that you are looking at. They also take this data and write more software to do geocoding and search. This is the process by which they take the text that you type in and figure out what data this refers to. This can be very complicated. While Google is probably the best at this, globally-speaking, errors are still common due to the wide range and complexity of what people type in. Most map websites also provide routing (directions) by car and/or other means. This process first involves geocoding to find the locations, and then routing software to use the vector data to find the best way between the locations.
Because of different vector and raster data and cartographic styles, online maps can look quite different. You can compare them at Map Compare and transparent map comparison. You can also use the GeoHack tool (linked below) to view many different maps and other geodata by inputting geographic coordinates.
The overhead images shown on map websites is taken from satellites (for the zoomed-out views) and airplanes (called aerial or orthographic images, for the zoomed-in views). These images are taken by many different companies and governments, and companies such as Google and Microsoft (owner of Bing) acquire them from all these different sources and do various processing on the images to merge them. Because the images were taken on different days by different cameras at different heights and zoom levels, the merged images often look patchy. Generally speaking, the quality of imagery is far higher in larger cities than in rural areas due to demand. Some websites also include oblique photos, taken from an airplane but at an angle instead of straight down. This has the advantage of being better able to see the sides of buildings, not just their roofs. On Bing these are mislabeled as ”Bird's eye„. A small number of websites including Google, Bing, and MapJack also include street-level photos taken by cars or other vehicles on the ground, allowing people to see what people see.
People have always struggled to display the spherical earth on a flat piece of paper or screen which is why there are many different map projections used. The most popular maps websites use a Mercator projection which has a lot of advantages for online maps but also seriously distorts the poles.
Wikipedia is a great source of information about places. A lot of the information there originated from the World Factbook (linked below), an annual publication by the CIA. Wolfram|Alpha is a great tool for looking at and comparing places and statistics, among other geographic and general purposes.
GIS, or Geographic Information System is the set of tools and data used by the mapping/cartographic industry. There are tons of vector and raster data available on the internet, some of which can be found using the tools on this page. This data may be available in a wide variety of formats such as the more traditional shapefile or the newer KML. The Google geodata search above simply searches Google for your keywords but only includes some common geodata file formats. A good list of US state and Canadian province offical geodata websites can be found on the GIS Spatial Data Clearinghouse Directory. There are so many different formats that it is often useful to be able to convert between them, so I have included links to GPSBabel below.
To view geographic data you can use any of a variety of GIS software. Some map websites such as Google and Bing allow you to enter in the the URL of a geodata file (usually in KML/KMZ or GeoRSS) directly to view it there. There are also two excellent software programs that aren't traditional GIS software and are essentially ”geo browsers„ or virtual globes: Google Earth (formerly Keyhole) and World Wind (built by NASA).
Some tools and websites including Google Earth and World Wind also include maps of the sky and other planets. You can also see Google Sky online or download WorldWide Telescope by Microsoft, which is also integrated into Bing Maps.