Satellites are used to collect information on fires. The data do not detect actual fires but rather indicators of fires, such as heat and smoke.

At the global level, there are two main data sources:

1. NASA operates two satellites which collect near-real-time data on active fires. VIIRS is a high-resolution satellite but it’s data only goes back to 2012. MODIS is a lower resolution satellite but has a longer time series, going back to 2001. The European Union’s Global Wildfire Information System (GWIS) also uses data from the VIIRS and MODIS satellites to detect active fires. 
2. The US National Oceanic and Atmospheric Administration (NOAA) has its own satellite and reports separately from NASA.

The commonly cited Global Forest Watch data uses a combination of information from the NASA and NOAA satellites to track fires. In Brazil, the national space institute (often referred to by its Brazilian acronym, INPE) uses data from the NOAA. Indonesia does not have a national space institute and therefore Global Forest Watch Fires data can be used to track fire alerts there.

At a global level, there are two principal datasets.

1. Global Forest Watch uses data from a combination of NASA and NOAA satellites to detect tree cover loss on a yearly basis and is often cited. It is convened by the World Resources Institute think-tank.
2. The UN Food & Agricultural Organisation (FAO) also collects the Global Forest Resources Assessment (FRA). This is generated through governments’ self-reporting on land that they classify as productive forest and assesses levels of forest cover on that basis. It also considers forest condition, management and uses. 

 More detail on the difference between the two sources can be found here and here.

In Brazil, two deforestation data sources are used:

1. The data from the national space institute (INPE) is one of the longest-running datasets. INPE runs two systems for monitoring forest loss. The first is PRODES which uses data from the Sino-Brazilian CBERS and Indian RISR-2 satellites to measure annual deforestation. The second is DETER which uses sensors from the same two satellites to detect vegetation degradation, mining and logging on a monthly basis. The Mapbiomas is a collaboration between NGOs and technology companies which verifies and refines deforestation alerts from DETER as well as monitoring systems for additional Brazilian biomes outside the Amazon.
2. IMAZON is an independent Brazilian research institute which runs the deforestation alert system. Established in 2007, it produces monthly forest loss figures.

The PRODES data differs from the Global Forest Watch (GFW) data in two ways. Firstly, PRODES measures on an annual cycle from July to August while GFW measures from January to December. This means that the two datasets collect data at different stages in the deforestation process. Secondly, the two systems collect data at different scales, in terms of hectare of area affected. This means that their figures for annual deforestation loss can differ. You can read more about the technical differences here.