Some reasons for the building boom are outlined in this page about larger changes happening across the entire bay area. Please help improve that page to provide a better picture of the changes happening.
See the construction page for areas where you're most likely to see new buildings going up.
What is being built?
Effects on the city
There are many! New condos are typically luxury condos, so most new residential buildings are not affordable for anyone but the wealthy. Housing shortage, displacement, and gentrification are huge issues right now and there's a lot of disagreement in the press, on social media, etc., as to whether new building projects are helping ease the housing shortage and/or whether they are increasing gentrification and displacement.
The city will certainly be taller after the current pipeline of towers gets built.
Apparently the building boom is affecting the rat population of the city.
Who is driving the building boom?
Because there's a huge housing shortage in the city and rents and home prices are astronomical, land is very valuable per square foot and developers are almost guaranteed high returns on investments. Additionally, mayor Ed Lee is a big proponent of construction. He is very friendly to the tech industry (tech industy people heavily financed his campaign) and he supports policies that make it easier for companies to move to and start new businesses in SF. He's also publically stated that building as fast as possible is the solution to the housing crisis and in 2014 he started advocating a plan to build 30,000 new housing units in the next six years. Additionally, many companies (especially tech companies) are now clamoring to locate their offices in the city. While previously it was more common for tech companies to locate their headquarters in Silicon Valley, companies have discovered that apparently employees (and potential employees) like living in SF more than in SV (who could've guessed?!). It's also a status thing to have your office right in the middle of the city or to put your name on the tallest new skyscraper currently under construction. Finally, because there's so much investment in tech right now and many tech companies are rapidly expanding, and tech company employees are currently in high demand, meaning that they are very well-paid, there are many people (both existing residents and people who move here from other places) with money to spend on expensive new housing.
Learn more about the building boom
For ongoing coverage, CurbedSF has lots of info about planned and current building. The San Francisco Business Times is also a great news source for goings on in SF capitalism. New Construction San Francisco is a blog run by a real estate company and it also has some coverage. Please add more sources including sources for understanding the current boom in the context of SF history here!