Most historians consider there to be two periods of British empire: the first starting as early as 1583, with the colony in Newfoundland, and the second being formed in the period 1783-1815, after the loss of the American colonies but continuing through Napoleon's final defeat. The "imperial century" of 1815-1914 saw major gains in British territory in Asia particularly, with the Raj in India being an empire inside an empire.

The major distinction between the two periods is seen as being the attention Britain paid to the Americas versus Africa and Asia.

Now, calling this a "British" empire is a bit tricky at times, because the United Kingdom wasn't formed until 1707; before that, the American colonies were English ventures, and Scotland itself attempted to establish a colony at Darien in modern-day Panama. (The failure of the Darien expedition bankrupted Scotland and is one of the reasons it sought union with England, but I digress.) But this will hopefully provide at least a bit of background for your question; let me know if anything is unclear.