Risk ratios and odds ratios--what are they?

A risk ratio (sometimes called relative risk, both abbreviated to RR) and an odds ratio (OR) are ways of expressing a comparison between two proportions. For example, in this paper, there are two randomised groups in the trial, women in the lateral position and women in the supported sitting position in the passive second stage of labour. The authors wanted to compare the proportions of instrumental births, episiotomies and perineal suturing in the two groups to see if there was any effect due to maternal position. A risk ratio gives the relative probability or chance of something happening in one group compared with the other, while an odds ratio gives the relative odds. Do you know what the difference is between probability and odds?