Besides looking at the links in this guide and Googling, look at law review and academic articles and their citations to data sources. Also look for the webpages of organizations concerned with your particular topic.
For the Progressive Prosecution class where you will need to find county level data, you might want to try Learcat and FBI Crime Data Explorer. This guide tries to indicate where county-level data is available, so you can Control+F for the word county.
Data:
Observations or measurements (unprocessed or processed) represented as text, numbers, or multimedia. Individual pieces of factual information.
Data sets:
"A data set is an organized collection of data. They are generally associated with a unique body of work and typically cover one topic at a time." Indeed.
Example: (Risk and Protective Trajectories, Community Context, and Juvenile Recidivism, Florida, 2015-2018 (ICPSR 38599) -Click on Download button on the right to download entire dataset)
The raw data looks like this:
Some of the websites on this guide enable you to download .csv spreadsheets of data sets, and/or manipulate the data yourself on the website and make your own charts or graphs ("interactive"). Sometimes the websites will only contain static tables or graphs in reports or summaries. To see some tables of the above dataset for juvenile recidivism in Florida, click on Data Related Publications, and go to Kevin T. Wolff, et al., Dynamic risk trajectories, community context, and juvenile recidivism, 86 J. Crim. Just. (2023). The article has charts, such as this, showing probability or rearrest for juveniles in different risk groups.
Variable
A characteristic, number or quantity that can be measured or counted. Examples: Age, race, sex, number of people incarcerated, number of staff at a prison, number of different types of crimes committed, etc.
Example: Risk and Protective Trajectories, Community Context, and Juvenile Recidivism, Florida, 2015-2018 (ICPSR 38599) - click on Variables. Search for Age, what does AgeEnrl mean?
Another Example from Easy Access to Juvenile Court Statistics.
Statistic:
Data are individual pieces of factual information recorded and used for the purpose of analysis. It is the raw information from which statistics are created. Statistics are the results of data analysis - its interpretation and presentation, often reported in a table, graph, or chart.
Example: See the above chart showing probability or rearrest for juveniles in different risk groups
Another Example: National Incident-Based Reporting System Reports (BJS page) - look at NIBRS reports, and LEARCAT (custom views and datasets)
Codebook:
Provides an in-depth explanation of how to interpret information in the database, the methodology, and explains each variable. Example: Death Penalty Census Codebook - look at the definition of the field "gender". (Death Penalty Census Database)
For more on statistical terminology see: Australian Bureau of Statistics - statistical terms and concepts