These are all words for a talk given to an audience.以上各词均指讲话、发言。
speech a formal talk given to an audience指演说、讲话、发言：◆Several people made speeches at the wedding.有几个人在婚礼上讲了话。
lecture a talk given to a group of people to tell them about a particular subject, often as part of a university or college course通常指大学里的讲座、讲课、演讲：◆a lecture on the Roman army关于罗马军队的讲座◆a course/series of lectures讲座课程；系列讲座
address a formal speech given to an audience指演说、演讲：◆a televised presidential address总统的电视演讲
speech or address?用 speech 还是 address？
A speech can be given on a public or private occasion; an address is always public. * speech 指公开或私下场合的讲话均可，而 address 总是指公开的演讲：◆He gave an address at the wedding.
talk a fairly informal session in which sb tells a group of people about a subject指相当不正式的报告、演讲：◆She gave an interesting talk on her visit to China.她做了个关于她在中国访问的有趣报告。
sermon a talk on a moral or religious subject, usually given by a religious leader during a service指布道、讲道：◆to preach a sermon布道
Patterns
a long/short speech/lecture/address/talk/sermon
a keynote speech/lecture/address
to write/prepare/give/deliver/hear a(n) speech/lecture/address/talk/sermon