"Standard Chinese Mandarin" has been defined as "taking the northern dialect as standard, with beijing pronunciation as the standard pronunciation".

But Almost no one speaks standard Mandarin in daily life,every city and region has their own accent or dialect.So people in different places,they speak different Mandarin with local accent.By region,northerners speak Mandarin better than southerners.The most standard Mandarin is what you heard on the news.

普通话是指以北京音为基础音，北方方言为基础方言，以典范的白话文作为语法的现代汉语。它不是自然形成的，而是人为规定的。所以普通人讲普通话都会带一点地方方言的口音和表达习惯。在南方表现得尤为明显，在语音上，南方方言大部分没有儿化，所以会说”画画“、”聊天“而不是”画画儿“、”聊天儿“，轻声也比较少，所以“这里”、“上去”、“先生”的第二个字都会发原来的声调。在词汇上，也有鲜明的地方色彩，北方人说一个人糊涂虫，南方人会说一个人弄不灵清；读数字的时候，110北方人会读一百一十，南方人会读一百十；在语法上，南方还有”我有去过那儿“、”走先“这样的特殊表达。总之，即便发音是标准的，平时讲话跟在电视上、课堂上听到的还是很不一样的。我有个同事是个典型的例子，他的普通话已经达到了一级，但在生活中他会有意识无意识地把翘舌发成平舌。虽然课文录音是标准普通话，但总感觉不接地气，想要说一口地道的中文，还是有必要学一点带有地方特色的普通话。

This is a very interesting question. Actually there are many local languages in China and usually the local people just speak local language in their region, but they don't speak Chinese mandarin well.  When the people from different places need to communicate , they have to speak mandarin, then we can hear standard mandarin and weird  mandarin. 

Yes, of course it's different between cities and regions.
No one actually speak 100 percent "standard mandarin" in daily life.
When someone mentions about "standard mandarin",
it normally refers to what we use in the common education system.
It's a little bit (just a tiny little bit!) like the English of Oxford English Dictionary and Received Pronunciation in English speaking world.
So,  even the conception of "standard mandarin" could also be a little different among Mainland China, Taiwan and Singapore. : )

Usually when we say Chinese,we mean Putonghua/Mandarin,which is regarded as the standard Chinese.Atually I don't think Mandarin is a correct defination of Putonghua,but since Mandarin has been called all those years,we just accept it.

Most people in China can speak Mandarin now,especially for the younger generation.Though almost everyone can speak one or more kind of dialect,they speak Mandarin.

In fact,even if two Chinese just speak their own dialect,which can have a real difference in pronunciation,they could understand each other's words well.

So when you're learning Chinese,it's not necessary to worry about dialects,just learn Putonghua/Mandarin.

As for Cantonese,they are only used in Hongkong, Fujian Province,Makao and regions near by.Und people there can also speak Putonghua.