は is called 副助詞, 係助詞, the 「は」 topic particle, topic marking particle, or etc. は identifies a topic, 窓 is the topic, in this case. Here is a sample example conversation with "は". Aさん: 家に窓はある？ Are there windows in your house? Bさん: 家に窓はあります。 There are windows in my house. が is called 格助詞, the 「が」 identifier particle, etc. が indicates that the speaker wants to identify something unspecified. Here is a sample example conversation with "が", Aさん: 家に何がある？ What are in your house? Bさん: 家に窓があります。 Some windows are in my house. ●は Indicates Contrast オレンジは好きです。 As for me, orange is the one I like. オレンジが好きです。 I like oranges (more than other fruits). 「オレンジは好き」 indicates that the speaker has something he or she dislikes, except for oranges. ●が Indicates New Information おじいさんがいました。There was an old man(An old man is new information). おじいさんは山へ行きました。The old man went to a mountain(The old man isn't new information). Here are some explanations of the differences between は and が in English. https://www.wasabi-jpn.com/japanese-grammar/ha-vs-ga-five-points-you-need-to-know/ Here are some explanations of the differences between は and が in Japanese with pictures. https://jn1et.com/ha-ga/

が and は look like a pair of similar particles but in fact they are not. ”は“ is an additional particle which emphasizes or adds other meanings to 格助詞. は is one of 係助詞, a marker for emphasis. が is one of 格助詞, a marker for a so-called subject. Say, 格助詞 and 係助詞 are in different categories. So you should not directly compare が ａｎｄ は. は can follow other 格助詞, like に＋は＝には、と＋は＝とは、で＋は＝では。 However, when が＋は＝がは, then "が" disappears, and the がは turns to は. i.e. "家に窓はあります" = "家に窓がはあります” 1). 家に窓があります. A house has windows . 2). 家に窓はあります。A house does have the very windows. 2). is practically unnatural or hardly makes sense by this sentence alone because it is obvious that "a house has windows" and there is no reason to emphasize the fact.