It is highly unusual these days for private individuals to host cocktail parties. Cocktail parties are normally held by corporations or charities and are usually held in lavish hotel banquet halls that look, and probably are, very expensive. If you get invited to a cocktail party the invitation will usually indicate what kind of dress is required. Most people, regardless of what the invitation says, will dress for a cocktail party and more often than not the person that does not dress looks and feels alienated. So even if the invitation says informal dress that usually means a suit of some sort but anything more elaborate is unnecessary. If the invitation says formal attire, or coats and tails, then you know it is something you will need to rent a tuxedo for because that is what they are expecting. It is always a good idea to follow the dress instructions on the invitation but expect that at the very minimum you will be wearing a suit of some sort.
There is rarely a full meal served at a cocktail party and the very name itself indicates that there is no elaborate meal to be served. There is usually finger foods set out in an elaborate display case that will usually include a statue of some sort, usually of ice or chocolate, and probably a fountain. Once again these will just be finger foods as most cocktail parties have other reasons for being held so food is not of the utmost importance. It is standard for gatherings like this to offer complimentary water and soft drinks but alcohol is usually an extra charge. In life the real cocktail parties are cash bars. It isn't a sign of disrespect, it is just being practical.
Most of all people expect you to mingle at a cocktail party. If you do not like reaching out and meeting new people then avoid a cocktail party at all costs because they are major networking functions. Some business professionals look forward to cocktail parties as ways to meet new clients so be prepared to be introduced to a lot of people. If you are not the friendly type, or you do not have a business to promote and people to network with, the a cocktail party may not be for you.