Working in the field with methods like observation and interviews will produce a lot of notes- these are called field notes. H. Russell Bernard observes that there are four kinds of field notes: jottings, diaries, logs and field notes proper.
Jottings include quick notes that can act like a good memory aid later. For example, noting down few words regarding what transpired during observation session or interviews.
Diaries as the name suggests is more personal. It includes your reflections or even your feelings or reactions to certain events that have taken place in the field. In a field setting, you may not be able to share your own struggles and issues with your participants. These feelings may be channelized into your diary.
Logs is an inventory of people you met (their designation, contact details etc), places you visited, how resources such as time and money were spent as well as to-do lists and list of things that were accomplished.
Field notes proper is also known as ‘writing up’. Bernard further divides field notes into methodological notes (notes on what methods worked for you as a while gathering data in the field as well as which ones did not, and why), descriptive notes (detailed descriptive accounts that are a result of observation in natural settings as well as interviews) and analytic notes (your inference and understanding of a particular community and their activities).
Apart from written descriptions, notes can include photographs, sketches, records and documents from government agencies and other materials which can trigger the memories of context later on, during the stage of data analysis. You could use traditional tools like a pen and book or even cell phone based applications like Evernote where you can take a picture, a video and annotate it with text and it is saved to cloud and can be accessed from anywhere.