These are all words for a person who talks or who is talking, especially in a particular way.
speaker a person who is or was speaking; a person who speaks a particular language; a person who makes a speech: I looked around to see who the speaker was; a fluent Arabic speaker; the keynote speaker at the conference lecturer a person who gives a lecture (= a talk to a group of people to teach them about a particular subject) communicator (rather formal) a person who is able to describe their ideas and feelings clearly to others: The ideal candidate will be an effective communicator. talker a person who talks in a particular way or who talks a lot: He’s a very persuasive talker; She’s a (great) talker (= she talks a lot).
SPEAKER OR TALKER? Talker is used when you are talking about how much somebody talks or how well they talk. It is not used for the person who is or was talking. You can say that somebody is a good/persuasive speaker but that means that they are good at making speeches. If you mean that they speak well in conversation, use talker.
conversationalist (rather formal) a person who is good at talking to others, especially in an informal way.
PATTERNS AND COLLOCATIONS • a good speaker/lecturer/communicator/talker/conversationalist • a(n) great/effective/skilful/excellent speaker/communicator • a poor speaker/lecturer/communicator/ conversationalist • a public speaker/lecturer • the keynote/main/principal speaker/lecturer