An apéritif is a pre-lunch or dinner drink that stimulates the appetite and prepares the palate for food. Traditional choices include chilled dry sherry (sipped from a small stemmed glass), vermouth (fortified white wine served over ice in a tall glass) and Campari (served on the rocks or with orange).
Spirit based drinks are also suitable; a cocktail, gin and tonic or a vodka and mixer. A simpler, and popular, option is a glass of chilled white wine or champagne. When offered an aperitif as a guest, check what other people are drinking or ask.