From my experience, there are multiple causes of the symptoms of waswas.
As described in the previous response, literally, the word "waswas" refers to the whispering of Shaytan. However in my experience, people also experience the same symptoms as:
* Psychological coping mechanisms (for instance, as a response to prolonged stress such as warfare or the pressure of being a minority and feeling one's identity is under threat)
* Manifestations of psychiatric conditions (anxiety, OCD, etc.) that are expressed in the guise of being "religious". Sometimes because this is considered to be a more socially acceptable way of expressing this behaviour or even praised and encouraged as signs of piety.
* Sometimes as a response to a very strict upbringing or social environment in which one is made to be terrified of small mistakes (like missing a spot during wudu), or where there is a lot of criticism of these things.
* I think to some degree there is a personality element as well.
Not all people who are dealing with these things respond in the same way (i.e. with symptoms of waswas); however, some do. Perhaps part of it is a desire for control in situations where we cannot control what is going on around us.
This does not mean that whispering from Shaytan is not present in these situations or that everything has a psychological/psychiatric explanation; rather, the two can happen at the same time.
Anyway, in addition to addressing what is due to Shaytan, such as by reciting ayat al-kursi, and the other interventions mentioned below, these things can also be addressed, if they are applicable in the circumstances.
Some people are also just naturally very scatterbrained and absent-minded and easily forget things (such as whether they ate or what time they have to go places, as well as things such as whether they did salat or wudhu), but this is somewhat different because it does not usually cause distress. However, some of the same interventions may apply (such as ignoring doubts).
In any case, as discussed in the previous response, the important thing is to overcome these things and acknowledge that this is not what Allah wants from us and rather the ritual acts we do such as salat and ritual purification are means to a higher goal rather than something to become anxious about in and of themselves.