Typically, the chalk talk will be restricted to faculty only, since you will be presenting your future research ideas and vision for your lab. Although it starts out like a normal talk, you should expect to be interrupted frequently. Although most of the faculty will have seen your job talk on the previous day, you can't assume everyone did (or that everyone remembers the key points). Any really important points from your job talk that are crucial to understanding your future research will have to be BRIEFLY reviewed (emphasis on the briefly). Your goal in the chalk talk is to describe what you envision doing as a researcher. You should describe the scientific problem and then explain your approach. You need to be really clear on the scientific problem, and also describe your approach.
Your audience will ask lots and lots of questions. Some will be really easy and some will challenge your science. You need to answer all of them to the best of your ability respectfully. DO NOT get defensive. Saying "I don't know" or some variant is better than trying to BS your way through something--there will probably be someone(s) who are also well-versed in the field. The department will be watching to see how you think on your feet, how well you have thought things through, and how you interact with your potential colleagues.
You need to be able to answer the following questions:
1. Where will I look for funding for this project (in general is OK--is it NSF/DoE/DoD fundable?)
2. What will happen if the project doesn't work? This is important--is there important science to write up along the way, or is the project only publishable if everything works as planned?
3. What are other groups doing in this area? Why is your approach unique/better?
4. If this is a continuation of prior work, will you be in direct competition with your mentor for funding? If so, why would agencies fund you and not your mentor?
5. What do you need to do the work equipment and space-wise, and how much will it cost (ballpark is OK)?
6. What are your initial staffing needs? Will you need a tech? How many students/postdocs will you look to get in the first 2 years when you are running on startup?
You should also be able to describe your target steady state group size, how quickly you can get started if you were to get an offer, and what mix of students/postdocs/techs are you looking for. Your research plans should include things that are short term, medium term, and long term. This is really important--you need to convince the department that you have a research agenda that is sustainable for 10+ years. If your proposed project is amazing, but it will be more or less complete when done, you need to know what will be next. You should plan on describing 3 projects, 2 in detail and 1 in outline due to time limitations, but be prepared with details on all 3 just in case. It is also a good idea to point out potential collaborations in the department and/or at the University if it is a natural extension of your proposed research.
They will probably also ask you what you want to and/or are prepared to teach in the department. This is not a deal breaker, but you should at least give it a little thought--you are applying to work at a University!