Defining your brand’s voice isn’t a one step process. It is, however, a crucial element of branding. In a nutshell, brand voice is the pillar for all communications. It determines the personality of your brand—by way of the words, phrases, and tone used on on your website, social media, and products—and sets the tone for each and every connection with your audience. Sticking to brand guidelines gives customers (or potential customers) a concrete sense of a brand.
Not sure where to start? Salesforce focus on creation of a brand's voice, style, and customized dictionary to create initial branding guidelines. Here is their excellent infographic 'How to Create a Succinct and Effective Brand Voice, Tone, and Style.' This is a must-read for all brand and marketing managers: