Here's a handy guide on several terms that have come up a lot in the local tech press lately:
Accelerator
"Accelerators provide structured curriculum in a short period to help rapidly grow the size and value of a company to get ready for a specific goal, typically to raise financing." — Blair Geisen
Most accelerators follow the model pioneered by TechStars and Y Combinator with an initial investment of $15,000-20,000 in exchange for 6-10% equity in the startup, an intense 90 day program that ends with "Demo Day" presentations in front of potential investors.
Examples: Iowa Startup Accelerator, the proposed Global Insurance Accelerator, TechStars, Gener8tor, .
Incubator
"Incubators provide guidance and advice to help startups grow and succeed in an unstructured program, with no specific goal or timeframe." — Blair Geisen
The concept of incubators predates accelerators and the two terms are often incorrectly conflated. Incubator structures vary greatly.
Examples: the former StartupCity Des Moines, West Des Moines Business Incubator, UNI's Innovation Incubator
Coworking Space
"The idea is simple: independent professionals and those with workplace flexibility work better together than they do alone. Coworking spaces are about community-building and sustainability. Participants agree to uphold the values set forth by the movement’s founders, as well as interact and share with one another. We are about creating better places to work and as a result, a better way to work." — Coworking Wiki
Examples: Veel Hoeden, Open Decorah, IC CoLab, Vault Coworking, Gravitate
Entrepreneurial Center of Gravity
"Everywhere you go, people are working on tech startups. They’re thinking about tech startups. There are investors and press and government officials and other people coming through to meet with the startups, and it’s a community center where there are meetups and events and classes and all kinds of other things going on to support that." — Joshua Baer
"There is no question in my mind that getting the startup community located within close proximity of each other has enormous benefits. The obvious starting point is the cross-pollination of ideas and talent. Importantly by locating a large number of startups in a single building or region it makes it easier for people passing through town to come to one place and meet with or speak to a large number of startups." — Mark Suster
Any of the three programs above (accelerators, incubators, coworking spaces) have the potential to become an entrepreneurial center of gravity—so long as the community gravitates (hey, that's a great idea for a name for such a place!) to it.
Examples: Gravitate, Geonetric's Headquarters, 1871, Capital Factory