Web of Science defaults to searching all databases in its collection.
If you wish to be more focused and exclude Medline (which is not a citation database) then chose to search Web of Science Core Collection.
Search on Web of Science will return records formatted as below, with the number of Times Cited given at the end of each record. You can sort results by Times Cited to find the most cited articles in a research area. Clicking on the number of Times Citated will open a full list of those citations.
A search on Scopus will produce results formatted as below. You can sort results by Cited by to find the most cited articles in a research area. The citations column on the right indicates the number of times the article has been cited since it was published. By clicking on the number of citations a full list of those citations will open.
You can also conduct more advanced analysis of the citations in your list of results (by Source Title, Author, Affiliation, Country, Document type or Subject area searches) by selecting Analyze Results.
View Citation Overview allows you to compare how different articles accrue citations over time.
View Cited by allows you to view the articles that have cited selected articles.
A search on PubMed will produce results formatted as below. The citations listed top right are related articles, the citations listed bottom right are other articles in the database that have cited this article. By clicking on a citation further information will open.