Dean Witter Discover & Co. said it agreed to acquire online stock broker Lombard Brokerage Inc., a San Francisco-based company that offers securities transactions via the Internet. The company said Lombard, founded in 1992, has won awards for its Web site and recently was named "the best online broker" by Barron's. Lombard conducts business in all 50 states and the District of Columbia. Both Dean Witter and Lombard declined to disclose terms of the transaction. Eric Roach, Lombard's founder, will remain with the company as chairman and chief executive. A spokesman for Dean Witter said Lombard will be "totally separate and distinct" from Dean Witter's other main business. Under the most recent pricing schedule announced by Lombard, customers willing to trade electronically were being charged $14.95 to trade up to 5,000 shares for exchange-listed stocks and for Nasdaq issues.