The stress changes depending on what is used.
With topic marker は, the information that comes after it is what is important. The topic is often the part of a sentence that will be omitted if it is already understood from context. Think of it as "On the topic of X..."
が marks the subject, and stresses the word that comes before it.
Both of these say "I am (name)" but their stress is different.
私は(Name)です - "(on the topic of me) - I am NAME" Stresses what your name is
私が(Name)です - "I (am the one who is) name" Stresses that you are the person with that name.
魚は好きです - (On the topic of fish) - I like it This could be in response to the question "do you like fish?"
魚が好きです - Fish (is the thing that I like) This could be in response to "What do you like?"
Swisdniak did a good job explaining the difference, but since this can be a tricky subject, I can provide more information and a few examples. This is a copy-paste of an answer I gave to some earlier today that asked a similar question in another thread. It explains the particles は, が, and を, as well as some basic sentence structure stuff:
"To mark the subject of the sentence, you would use が. The particle は is the topic marking particle. Frequently, the topic and subject overlap, but I think it is important to keep them distinct in your mind. If you start thinking of は as a subject marker, it will be really confusing when you start to encounter sentences where the topic is NOT the same as the subject. Or sentences where the subject in the English translation is different from the grammatical subject in the Japanese sentence, which happens more often than you might expect.
を is the direct object marker. The subject is the one doing the verb's action. The direct object is the one that has that action done to them/it.
(Jon ga ringo o tabemasu.)
"John eats apples"
John is the subject.
Apple(s) are the direct object.
"To eat" or "eats" is the verb (action).
Therefore, John is the one doing the eating. The apples are the ones being eaten.
In English, sentence order determines grammatical relationships. We know that John is the subject, because he comes first. If we change the order, the meaning changes. "Apples eat John." But in Japanese, it is not sentence order that tells you who is doing what to whom ... it is particle placement. We know John is the subject because ジョン is followed by が and we know that apples are the direct object because リンゴ is followed by を. This would remain true, even if we reversed the word order, so long as the particle location stays the same.
(Ringo o jon ga tabemasu.)
"John eats apples."
However, you have probably noticed that が isn't used as much as you would expect for such an important core grammar marker. This is because Japanese tends to prefer leaving out the subject when it can be implied by context. Similarly, the language also rarely uses personal pronouns like "I, you, he/she, we, they, etc". There are words that can be used, like わたし and あなた, but it is not required and, depending on circumstances, can actually sound impolite or unnatural.
In situations where a subject would normally be used in English, Japanese tends to use topics to indirectly refer to what they are going to talk about. Directly stating the subject is usually limited to situations where it is unavoidable.
Referring back to the previous example, the more natural way to say it in Japanese would be:
(Jon wa ringo o tabemasu.)
"As for John, (he) eats apples."
The は is marking a new topic that the speaker would like to discuss. The current sentence and future sentences will be related to that topic. I will usually translate は to meaning something like "As for X" or "Speaking of X" or "Regarding X". The subject of the sentence, (he), is implied by this topic, but they not the same thing. Often times, the English translation will ignore the distinction between が and は, so this sentence could also be naturalized to "John eats apples."
Because は sentences are so often translated identically to が sentences, it is very common for new Japanese students to get them mixed up and not understand how they are different. And it is really easy to start thinking that は always marks the subject of the sentence, which is not true.
One classic example of this is:
(Zō wa hana ga nagai.)
"Elephants have long noses."
Now look at the Japanese. What is the subject of the Japanese sentence - Is it elephants? Nope. Elephants (象) are the topic, marked by は, but noses (鼻) are the subject, marked by が. So what is really going on here?
A more literal translation of this sentence would be "Speaking of elephants, (their) noses are long." The speaker is talking about elephants, but they are NOT the grammatical subject. This kind of sentence structure happens a lot more commonly in Japanese then it does in English, so when the sentence gets translated, the grammar frequently gets changed to provide a more natural-sounding English equivalent. Don't let the translation fool you. Trust the particles ... they don't lie.
I hope this helps to clear up some confusion."