There is a wealth of resources for learning Latin available on the internet. Annnotated texts are available for a number of the principal works of classical Latin literature. If you are having trouble in parsing a passage, an online search may provide the answers you are looking for. Reasonable caution is, of course, necessary. Some information, especially in chatroom-style sites, is unreliable.
A few of the most useful online resources:
The Latin Links page maintained by John Piazza provides links to online texts, online Latin learning tools, sites of general Latinitas interest, sites on which you can hear Latin spoken, and more. There is much on this page that is of practical use by the serious scholar and much also that anyone interested in Latin will enjoy.
The Perseus Project is an online library project maintained by Tufts University. The original focus was on classical Greek literature but the collection has expanded to include many texts in Latin and other languages. Latin texts offer word-by-word meanings and basic parsing data which pop up at a click and commentaries on a number of texts as well.
The Vergil Project, maintained by the University of Pennsylvania, is an immensely rich online resource on, unsurprisingly, Vergil and his works. Included is the entire Aeneid with word meanings and detailed parsing data for each word appearing in a separate window as readers click on words. There is also a substantial body of commentary.
Hard-copy dictionaries remain advantageous compared with online counterparts in containing more alternative meanings and explanations of variations and cases used with words. This said, the online Latin-English dictionary which appears to contain the largest selection of words, with reasonably complete meanings and a convenient look-up format, is William Whitaker's Words made available by the University of Notre Dame archives.