At times I am surprised that even persons dealing with Switzerland and with Swiss people sometimes are not aware of the language situation in Switzerland. To everyone else, obviously, this may be completely new:
In the French part, people use (the Swiss version of) FRENCH.
In the Italian part, people use (the Swiss version of) ITALIAN.
In the German part, people use (the Swiss version of) GERMAN to 5% and SWISS GERMAN to 95%, two separate languages (according to UNESCO, SIL, etc).
What does that mean?
A famous Swiss poet once explained:
The Swiss speak a language they don't write and write a language they don't speak.
GERMAN (or to be precise: SWISS STANDARD GERMAN) is
A. official language, together with French, Italian, Rhaeto-Romance
B. learned at school, from age of 6 years
C. written language, i.e. all media, books, documents
SWISS GERMAN is
A. mother tongue
B. spoken language
C. exists in a variety of (local) dialects. Best known ones are: Bündner Tütsch, Sanggaller Tütsch, Züritüütsch, Bärndütsch, Baslerdütsch, Wallisertiitsch
People living in Switzerland, with a German (or similar, e.g. Swedish, Dutch) background, will typically be able to understand both GERMAN and SWISS GERMAN after a short time (couple of weeks). The difference between the two language is however large enough (a tad bigger than between Italiano and Castellano) to be initially mutually unintelligible. This can be compared to the situation in China with Standard Chinese (Putonghua, sometimes called Mandarin) and local languages such as Shanghainese (Shanghaihua) or Cantonese (Guangdonghua), which are mutually unintelligible.
The reason for immediate incompatibility lies primarily in
A. vastly different grammar: different topology, tenses, conjugation,
B. different vocabulary / etymology
Accent plays a subordinate role here.
There is a clear division in the use of the two languages.
GERMAN for everything official and everything written.
SWISS GERMAN for everyday life, at work and at home, in meetings, phone calls, etc.
For most Swiss people GERMAN language use makes up 2-5% of their language use, and SWISS GERMAN 95-98%, obviously varying with their occupation.
OFFICIAL STATUS of the Swiss version of GERMAN:
GERMAN (we are talking about the Swiss version of Standard German, also called High German) is regulated, due to its official status, just as any other language is. While much is identical with the German version of Standard German, there are still differences; mostly in vocabulary, orthography, pronunciation.
People unaware of the situation - i.e. visitors, including most people from Germany - may occasionally mistake the Swiss version of GERMAN for SWISS GERMAN.
Swiss people do not often speak GERMAN, it is mostly used in written form. When they do, there is a strong and distinct accent noticeable.
DIALECTS in SWISS GERMAN
SWISS GERMAN, as the common language in Switzerland, is divided into distinct dialects, along geographic division lines. While all dialects in SWISS GERMAN share a linguistic basis, they may substantially differ in vocabulary and pronunciation, up to a degree which makes mutual understanding nearly impossible. Moderate dialects in the 'flatlands' (Swiss Central Plateau) are fully mutually intelligible.
Hope this helps as an overview.