Some questions would be helpful. Different odds forms: Decimal, Standard English... These are the most common.
e.g. A decimal odd of 3.0 would mean if you gave them £10, and won, you'd get £30 including your stake. However, the Standard English for this would be 2:1 , which expresses the ratio of profit to stake...
A lay bet is when you act as the bookie, essentially. You bet that and outcome will not take place, and you 'offer odds' in exchange for a certain stake...
A back bet is when you back an outcome to happen, the most common form of betting.
Back and Lay are almost the same, but not quite, in events in which there are only two possible outcomes.