Confusing Terms of CRM

One of my friend called me and was asking about some terms of CRM. Although not difficult but some what confusing for new comers. He is working on sugar CRM and have a problem in identifyng Lead, Opportunity, Account and Contact. Well the term lead is same as sales lead it is before the first contact with the person but he may be interested if contacted. Secondly Opportunity is a fair opportunity depending on the sales man, he had an open opportunity to make a cold opportunity or hot one. If it becomes cold then he missed the sales but if it is hot one. He is surely becoming a client or account for him. Account is basically the sales account it can either be a person or a corporate. If it is corporate you can create mulitple contacts for each account and a single one for person. Both contact and acount can be used for further sales and service jobs.