In the Roman mind, there was a sort of contract between the gods and the mortals. As part of this agreement each side would provide as well as receive services.
Thee role of the mortal in this partnership with the gods was to worship the mighty gods. For this there was prayer and sacrifice. And for both of these activities there was firmly defined rituals. To perform these ritual correctly was of paramount importance. One mistake and one would have to begin all over again. The very nature of Roman religion itself, with its numerous gods, many of which had multiple roles, was cause for problems. Particularly as in some cases not even the sex of a deity was clear. Hence the phrase 'wether you be god or goddess' was a widespread in the worship of certain deities. Many Roman gods also had entire colelction of additional names, according to what aspect of life they were a patron to. So, for example Juno was Juno Lucina in her role of goddess of childbirth. But as goddess of the mint she was known as Juno Moneta (this curious role came about because for a long time the Roman state mint was housed in her temple on the Capitoline hill).
There appear to have been few things for which there was not a special prayer. So, for example, there were prayers to bring bad luck upon another, or for the return of stolen property.
A prayer almost always will have been made together with a small offering to the deity. Such sacrifices did not always need to involve the killing of an animal, although this was very often the case. For the sacrifice had to be a symbol of life in some way or form. Milk, fruit, cheese, also wine were often used as less bloody offerings to the gods. But naturally for the official rituals of the state gods it was animals which most of the time were sacrificed. And for each god there woudl be different animals. For Janus one sacrificed a ram. For Jupiter it was a heifer (a heifer is a young cow which has not yet had more than one...