"教师"is an Occupation. "老师" is what we usually say. Nomatter 教师or老师，they are same thing. For example:A:What do you do,Lenny? B:I am a 教师.(or my occupation is 教师） A:Hey,Lenny.Do you know that girl in black? B:Oh!That's my 老师. Everything clear?

The following is my point of view： Both of the words can use in the spoken language or written language. First 教师 is： 1. career 2. one person or a group of people who teach students in the school or somewhere. Then 老师： 老师has the same meaning as 教师，but when you speak to a specific person or a group of people I prefer use 老师 not 教师： For example： 陈老师 is fine and 陈教师 is weird to me , ：） In the morning，when greeting，老师好 is fine，but 教师好 is weird, and I have never heard it before. You can say She is a 老师 or 教师。

老师与教师比,老师是口语,教师是书面语.但是"老师"意思更广泛,涉及各行各业,且能用来进行直接称呼,教师则不能,只能用于正规学校的职务职称用. The teacher is speaking, the teachers are written. But the "teacher" means a broader, involving all walks of life, and can be used for direct address, the teacher can not, can only be used with regular school duties title.

OMG!In fact,it's different between 教师and老师，u know he is my teacher=他是我的老师 not 他是我的教师。because 教师is a professional title,just like engineer and staff...老师 is compare to 学生（students）so 1。老师后面可以+带姓（first name）/名的指人名词or 人称代词（my、his、her、our and so on），such as王老师，王小刚老师，小刚老师，我的老师（my teacher）... 2。教师一般前面只能+量词such as： I am a teacher=我是一名教师。Teachers‘Day=教师节not 老师节，u know here teacher is a professional title. The teachers are stereotyped as "candles", "gardeners" and "engineers of souls" here the "teacher"is also "教师"not 老师

when you are talking about “teacher”, they are of no difference. 老师 is more used orally but when you want ask somebody if he is not a teacher you can use 老师 to show your respect