The activity of a source is defined as the rate at which a source of unstable nuclei decays measured in decays per second.
The unit for activity is the Becquerel (Bq) where 1 Bq = 1 decay per second.
The decay could result in a particle being emitted (normally alpha, beta, gamma or neutron) or the emission of a photon of electromagnetic radiation (gamma ray).
The activity of radioactive sources held by Physics departments in schools in the UK is usually about 150 kB (150 000 Bq) and the background radiation about 0.5 Bq.
The activity is the total number of emission per second in all directions from the source taken. However if you use a Geiger tube (or any other detector) to measure the number of emissions you will only record a fraction of the total emissions. This is because normal detectors do not usually surround the source and so you only detect particles (or photons) emitted into a small angle (see diagram). This means that the observed count rate is always much less than the activity of the source.
Count rate is also measured in Bequerel.
