Russian history has no shortage of peasant and cossack rebellions of different nature, and Pugachev's rebellion, despite being one of the biggest (or even THE biggest) in terms of numbers and continuity, shares many similar characteristics with earlier uprisings, sometimes even with similar insurrections in other countries, such as Bogdan Khmelnitsky's rebellion which started in 1648.
Reasons for both cossacks and serfs - most numerous groups of participants - were very much alike, although not necessarily intersecting.
Cossacks were a quasi-ethnic, primarily warrior subgroup living on Russian and Polish frontiers, in 18th century vast majority of Cossack settlements came under Russian jurisdiction. Russian emperors have generally been very strict in enacting new laws and restricting freedoms of different cossack voiska (literally "army", but more in a feudal meaning of the word, not standing regular army per se) since by the time Russa had turned into great empire they no longer possessed any need for hardly controllable, semi-independant people of the steppe frontier, and rightfully feared that their excessive freedom would lead to regional instability. Plus, they wanted to gain absolute control over territory occupied by cossacks and gain as much as possible, while spending less - for example, they would lift tax relaxations and demote cossacks to the status of regular imperial citizen, and in 1754 a state monopoly on salt trade was enacted (it was a heavy blow for Yaitzk cossacks because they largelly depended on fish trade, and fish is hard to preserve without salt). They would also try to appoint cossack atamans (warchiefs) in place of elected ones to gain better control and be on favourable condition when enacting new laws and taxes upon them. Similar reasons were at play in 17th century Poland when Ukrainian cossacks seemingly had enough of ever-growing list of restrictions and budget cuts of their Polish masters and eventually rebelled. Note that, just like in Bogdan's rebellion, not everybody in Pugachev's army was of lower class as many cossacks had some ties with nobility (just like Bogdan was de facto a member of Polish nobility despite leading a "commoner" revolt).
Serfs would also face very harsh freedom restrictions during this time as with passing years Russia would tighten the grip around peasant's neck in order to increase their economical efficiency and please old-fashioned Russian nobility. Catherine II's 1767 decree outright stated that peasants' point of view was out of the question when dealing with their masters' wrongdoings - essentially it completely disenfranchised the commoners. Rapid industrial development of Urals which possessed large amounts of valuable resources faced with lack of manpower at early stages - it was evident that in order to operate, dozens of big factories needed thousands of working hands. To solve this problem, many serfs were removed from their lands and put to work in mines and manufactories, sometimes entire settlements. With their deteriorating working and living conditions (remember that land still had to be plowed so remaining peasants in many villages would be pressed to work even more) and complete inability to communicate with the government, commoners started facing intensified abuse from their masters who had found out there no longer were any repercussions for violent behaviour towards their serfs.
Ethnic minorities of Siberia and closely related territories (kalmyks, bashkirs in others) were also very dissatisfied with fast industrialization and de facto colonization of Urals, because their freedoms were also partially restricted and they had a feeling like their native lands would soon be plundered and enslaved by the state. They would also take active roles in the rebellion in hopes to make Russian government ease the restrictions or probably abandon their development plans altogether.
Experience of peasants participating in rebellion was probably just like they imagined it would be, as many nobles caught unaware in their estates would be easily overpowered and face grim death at the hands of the oppressed. However, once it became clear Russian government wasn't going to back off and was determined to suppress the revolt, peasants' morale started wavering. It is important to understand that, despite de jure similar aims and reliance upon each other, different groups participating in the insurrection didn't trust each other fully. Cossacks ironically considered commoners to be unworthy of freedoms they hoped for, and didn't want serfs flocking on cossack lands in order to become frontier people (a dream for many runaway peasants) - they thought of themselves as of somewhat isolated caste which was welcoming to outcomers but, at the same time, opposed active colonization by any other group of men. Roughly the same sentiments were true for the Ukrainian revolts in 17th century as cossacks tried to use peasants as some form of a meat shield to better suit their needs without taking excessive losses, and once serfs understood that they no longer had desire to help their allies likewise. So in any way, generally serfs' experience was miserable as they were considered to be entirely disposable by other participants.
After state armies had destroyed rebel hosts in several engagements and Pugachev himself had been executed, rebellion was mostly suppressed. Many peasants participating in the revolt would be tortured and killed for obvious reasons, while others would be put back to work. Despite lifting some of the state monopolies and changes of cossacks' status (they would soon be fully transformed into full-scale army units on the state's service), overall serf positions weren't drastically improved and their essentially slave-like conditions will end only in the second half of the 19th century.
Sources: